fix to make it work with WPG2 plugin & other php-generated images calls.

An update :

function tb_post_thumb($generate=false, $alt_text='', $resize_width = 0, $resize_height = 0, $crop_x = 0, $crop_y = 0) {

	global $post;

	$settings = get_option('post_thumbnail_settings');

	// dir to save thumbnail to

	$save_dir = $dest_path['dirname']."/{$settings['folder_name']}"; //GALLERY2 MOD : this has been moved from above

	// find an image from your domain

	if (preg_match('/<img (.*?)src="https?:\/\/(www\.)?'.str_replace('/','\/',$settings['domain_name']).'\/(.*?)"/i',$post->post_content,$matches)) {

		// put matches into recognizable vars

		// fix later, assumes document root will match url structure

		$the_image = $settings['base_path'] . '/' . $matches[3];

		///GALLERY2 MOD - IMGS TAGS

		$the_image_url = $settings['full_domain_name'] . '/' . $matches[3];

		$the_image_url = str_replace ('&amp;','&', $the_image_url);

		$save_path = $settings['base_path'].$dest_path['dirname']."/{$settings['folder_name']}";

		$g2_pattern = "g2_itemId=";

		$g2_image = explode($g2_pattern, $the_image);

		if(!empty($g2_image[1])) { //G2 MOD

			$g2_id = explode('&', $g2_image[1]);

			$g2_id = $g2_id[0];

			$the_image = htmlspecialchars_decode($the_image);

			$ch = curl_init(); // create cURL handle (ch)

			if (!$ch) {

			    die("Couldn't initialize a cURL handle");

			}

			// set some cURL options

			$ret = curl_setopt($ch, CURLOPT_URL,            $the_image_url);

			$ret = curl_setopt($ch, CURLOPT_HEADER,         1);

			$ret = curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 0);

			$ret = curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

			$ret = curl_setopt($ch, CURLOPT_TIMEOUT,        30);

			// execute

			$ret = curl_exec($ch);

			$content_type = curl_getinfo( $ch, CURLINFO_CONTENT_TYPE );

			curl_close($ch);

			if (preg_match("/image\/png/", $content_type)) {

				$new_ext = '.png';

			}elseif (preg_match("/image\/gif/", $content_type)) {

				$new_ext = '.gif';

			}elseif ( (preg_match("/image\/jpg/", $content_type)) || (preg_match("/image\/jpeg/", $content_type)) ) {

				$new_ext = '.jpg';

			}elseif (preg_match("/image\/bmp/", $content_type)) {

				$new_ext = '.bmp';

			}elseif (preg_match("/image\/tiff/", $content_type)) {

				$new_ext = '.tif';

			}			

			if ($new_ext) { //si le fichier est une image

				$save_url = $settings['full_domain_name'].$dest_path['dirname']."/{$settings['folder_name']}";

				$the_new_path = $save_path.'/wpg2-'.$g2_id.$new_ext;

				$the_new_url = $save_url.'/wpg2-'.$g2_id.$new_ext;

				$the_new_thumb_url = $save_url.'/.wpg2-'.$g2_id.$new_ext;

				$the_new_thumb_path = $save_path.'/.wpg2-'.$g2_id.$new_ext;

				$the_image = $the_new_thumb_path;

			}

		}

		if ((!file_exists($the_image)) && (!$the_new_thumb_path) ) {

			return tb_post_thumb_gen_image	(

												str_replace($settings['full_domain_name'],$settings['base_path'],$settings['default_image']),

												$settings['default_image'],

												$alt_text,

												$generate

											);

		} elseif ($the_new_thumb_path) {

			if (!file_exists($the_new_thumb_path)) {

				if (copy($the_image_url, $the_new_path)){

					chmod ($the_new_path, 0777);

					$the_image = $the_new_path;

				} else {

					unset($the_new_path);

					echo "probleme lors de la copie";

				}

			} else {

				return tb_post_thumb_gen_image	(

													$the_new_thumb_path,

													$the_new_thumb_url,

													$alt_text,

													$generate

												);

			}		

		}

		$dest_path = pathinfo($the_image);

		// name to save to

		if ($settings['append'] == 'true') {

			$filename = substr($dest_path['basename'], 0, strrpos($dest_path['basename'], "."));

			$rename_to = $filename.$settings['append_text'].'.'.$dest_path['extension'];

		}

		else $rename_to = $settings['append_text'].$dest_path['basename'];

		// check if file already exists

		// return location if does

		if (file_exists($save_path.'/'.$rename_to)) {

			return tb_post_thumb_gen_image	(

												$save_path.'/'.$rename_to,

												$save_dir.'/'.$rename_to,

												$alt_text,

												$generate

											);

		}

		// sticky bit?

		if (!is_dir($save_path)) mkdir($save_path,02775);

		// manipulate thumbnails

		$thumb = new ImageEditor($dest_path['basename'],$dest_path['dirname'].'/');

		$thumb->resize($settings['resize_width'], $settings['resize_height'], $settings['keep_ratio']);

		if ($settings['crop_exact'] == 'true' || ($crop_x != 0 && $crop_y != 0)) {

			if ($crop_x != 0 && $crop_y != 0) {

				$settings['resize_width'] = $crop_x;

				$settings['resize_height'] = $crop_y;

			}

			if ($thumb->x > $settings['resize_width' || $thumb->y > $settings['resize_height']]) {

				$thumb->crop(	(int)(($thumb->x - $settings['resize_width']) / 2),

								0,

								$settings['resize_width'],

								$settings['resize_height']

							);

			}

		}

		$thumb->outputFile($save_path."/".$rename_to, "");

		if ($the_new_path) { //G2 MOD

			unlink($the_new_path);

		}

		return tb_post_thumb_gen_image	(

												$save_path.'/'.$rename_to,

												$save_dir.'/'.$rename_to,

												$alt_text,

												$generate

											);

	} else {

		if (!empty($settings['video_regex']) && tb_post_thumb_check_video($settings['video_regex'])) {

				$settings['default_image'] = $settings['video_default'];

		}

		return tb_post_thumb_gen_image	(

											str_replace($settings['full_domain_name'],$settings['base_path'],$settings['default_image']),

											$settings['default_image'],

											$alt_text,

											$generate

										);

	}

}

Here are my plugin settings, for information :

WORDPRESS PAGE : http://monsite.domain.com/wordpress

DOMAIN NAME: monsite.domain.com
DEFAULT IMAGE : http://monsite.domain.com/wordpress/wp-includes/images/post_thumbs/post_nopic.png
FULL DOMAIN NAME : http://monsite.domain.com
BASE PATH : /home/monsite/public_html
FOLDER NAME : wordpress/wp-includes/images/post_thumbs

some fix

function tb_post_thumb($generate=false, $alt_text='', $resize_width = 0, $resize_height = 0, $crop_x = 0, $crop_y = 0) {

	global $post;
	$settings = get_option('post_thumbnail_settings');
	// dir to save thumbnail to
	$save_dir = $dest_path['dirname']."/{$settings['folder_name']}"; //GALLERY2 MOD : this has been moved from above

	// find an image from your domain

	if (function_exists('g2_imagebypathinpost')) { //!!!because we can't use  "add_filter('get_the_content', 'g2_imagebypathinpost', 0);" into wpg2.php, see http://codex.wordpress.org/Plugin_API/Filter_Reference
		$post_content = g2_imagebypathinpost(get_the_content());
	} else {
		$post_content = get_the_content;
	}

	if (preg_match('/<img (.*?)src="https?:\/\/(www\.)?'.str_replace('/','\/',$settings['domain_name']).'\/(.*?)"/i',$post_content,$matches)) {
		$the_image = $matches[3];
		if ( ($resize_width != 0) ||($resize_height != 0) ) {
			$the_dimensions = '-';
			if ($resize_width != 0) {
				$the_dimensions .='W'.$resize_width;
			}
			if ($resize_height != 0) {
				$the_dimensions .='H'.$resize_height;
			}
		}

		// put matches into recognizable vars
		// fix later, assumes document root will match url structure
		$the_image = $settings['base_path'] . '/' . $matches[3];

		///GALLERY2 MOD - IMGS TAGS
		$the_image_url = $settings['full_domain_name'] . '/' . $matches[3];
		$the_image_url = html_entity_decode($the_image_url);

		$save_path = $settings['base_path'].$dest_path['dirname']."/{$settings['folder_name']}";
		$g2_pattern = "g2_itemId=";
		$g2_image = explode($g2_pattern, $the_image);

		if(!empty($g2_image[1])) { //G2 MOD

			$g2_id = explode('&', $g2_image[1]);
			$g2_id = $g2_id[0];
			$the_image = htmlspecialchars_decode($the_image);

			$ch = curl_init(); // create cURL handle (ch)
			if (!$ch) {
			    die("Couldn't initialize a cURL handle");
			}
			// set some cURL options
			$ret = curl_setopt($ch, CURLOPT_URL,            $the_image_url);
			$ret = curl_setopt($ch, CURLOPT_HEADER,         1);
			$ret = curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 0);
			$ret = curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
			$ret = curl_setopt($ch, CURLOPT_TIMEOUT,        30);

			// execute
			$ret = curl_exec($ch);

			$content_type = curl_getinfo( $ch, CURLINFO_CONTENT_TYPE );

			curl_close($ch);

			if (preg_match("/image\/png/", $content_type)) {
				$new_ext = '.png';
			}elseif (preg_match("/image\/gif/", $content_type)) {
				$new_ext = '.gif';
			}elseif ( (preg_match("/image\/jpg/", $content_type)) || (preg_match("/image\/jpeg/", $content_type)) ) {
				$new_ext = '.jpg';
			}elseif (preg_match("/image\/bmp/", $content_type)) {
				$new_ext = '.bmp';
			}elseif (preg_match("/image\/tiff/", $content_type)) {
				$new_ext = '.tif';
			}			

						if ($new_ext) { //si le fichier est une image
				$save_url = $settings['full_domain_name'].$dest_path['dirname']."/{$settings['folder_name']}";
				$the_new_name = '/wpg2_'.$g2_id.$the_dimensions;

				$the_new_path = $save_path.$the_new_name.$new_ext;
				$the_new_url = $save_url.$the_new_name.$new_ext;
				$the_new_thumb_url = $save_url.'.'.$the_new_name.$new_ext;
				$the_new_thumb_path = $save_path.'.'.$the_new_name.$new_ext;
				$the_image = $the_new_thumb_path;

			}
		}

		if ((!file_exists($the_image)) && (!$the_new_thumb_path) ) {
			return tb_post_thumb_gen_image	(
												str_replace($settings['full_domain_name'],$settings['base_path'],$settings['default_image']),
												$settings['default_image'],
												$alt_text,
												$generate
											);
		} elseif ($the_new_thumb_path) {
			if (!file_exists($the_new_thumb_path)) {
				if (copy($the_image_url, $the_new_path)){
					chmod ($the_new_path, 0777);
					$the_image = $the_new_path;
				} else {
					unset($the_new_path);
					echo "probleme lors de la copie";
				}
			} else {
				return tb_post_thumb_gen_image	(
													$the_new_thumb_path,
													$the_new_thumb_url,
													$alt_text,
													$generate
												);
			}
		}

		$dest_path = pathinfo($the_image);

		// name to save to
		if ($settings['append'] == 'true') {
			$filename = substr($dest_path['basename'], 0, strrpos($dest_path['basename'], "."));
			$rename_to = $filename.$the_dimensions.$settings['append_text'].'.'.$dest_path['extension'];
		}
		else $rename_to = $settings['append_text'].$dest_path['basename'];

		// check if file already exists
		// return location if does
		if (file_exists($save_path.'/'.$rename_to)) {
			return tb_post_thumb_gen_image	(
												$save_path.'/'.$rename_to,
												$save_dir.'/'.$rename_to,
												$alt_text,
												$generate
											);
		}

		// sticky bit?
		if (!is_dir($save_path)) mkdir($save_path,02775);
		// manipulate thumbnails
		$thumb = new ImageEditor($dest_path['basename'],$dest_path['dirname'].'/');

		if ($the_dimensions) {
			$thumb->resize($resize_width, $resize_height, $settings['keep_ratio']);
		}else{
			$thumb->resize($settings['resize_width'], $settings['resize_height'], $settings['keep_ratio']);
		}

		if ($settings['crop_exact'] == 'true' || ($crop_x != 0 && $crop_y != 0)) {
			if ($crop_x != 0 && $crop_y != 0) {
				$settings['resize_width'] = $crop_x;
				$settings['resize_height'] = $crop_y;
			}
			if ($thumb->x > $settings['resize_width' || $thumb->y > $settings['resize_height']]) {
				$thumb->crop(	(int)(($thumb->x - $settings['resize_width']) / 2),
								0,
								$settings['resize_width'],
								$settings['resize_height']
							);
			}
		}
		$thumb->outputFile($save_path."/".$rename_to, "");

		if ($the_new_path) { //G2 MOD
			unlink($the_new_path);
		}

		return tb_post_thumb_gen_image	(
												$save_path.'/'.$rename_to,
												$save_dir.'/'.$rename_to,
												$alt_text,
												$generate
											);
	} else {
		if (!empty($settings['video_regex']) && tb_post_thumb_check_video($settings['video_regex'])) {
				$settings['default_image'] = $settings['video_default'];
		}
		return tb_post_thumb_gen_image	(
											str_replace($settings['full_domain_name'],$settings['base_path'],$settings['default_image']),
											$settings['default_image'],
											$alt_text,
											$generate
										);
	}
}