TimThumb How to load only if image exists

I’ve got timthumb in the theme I’m using, but I want to only load if the image exists and if not, have it not display.

How would I go about doing this?

Owner of the code pointed me to this section of code.

function pong_posts($sort = 'recent', $items = 5, $echo = TRUE)
{
	$pp_blog_cat = get_option('pp_blog_cat');
	$return_html = '';

	if($sort == 'recent')
	{
		$posts = get_posts('numberposts='.$items.'&order=DESC&orderby=date&post_status=publish');
		$title = 'Recent Posts';
	}
	else
	{
		global $wpdb;

		$posts = $wpdb->get_results("SELECT comment_count, ID, post_title, post_content, post_date FROM $wpdb->posts WHERE post_type = 'post' AND post_status = 'publish' ORDER BY comment_count DESC LIMIT 0 , ".$items);
		$title = 'Popular Posts';
	}

	if(!empty($posts))
	{

		$return_html.= '<h4 class="widgettitle">'.$title.'</h4><hr/>';
		$return_html.= '<ul class="posts blog">';

			foreach($posts as $post)
			{
				$image_thumb = get_post_meta($post->ID, 'blog_thumb_image_url', true);
				$return_html.= '<li><a href="'.get_permalink($post->ID).'"><img src="'.get_bloginfo( 'stylesheet_directory' ).'/timthumb.php?src='.$image_thumb.'&h=60&w=60&zc=1" alt="" /></a><h6><a href="'.get_permalink($post->ID).'">';
				$return_html.= pong_substr($post->post_title, 50).'</a></h6><a href="'.gen_permalink(get_permalink($post->ID), 'quick_view=1').'" class="quick_view" title="Quick View"><img src="'.get_bloginfo( 'stylesheet_directory' ).'/images/icon_quick_view.png" style="width:16px" class="mid_align"/></a>'.date('F j, Y', strtotime($post->post_date)).'<br/><br/>';
				$return_html.= pong_substr(strip_tags(strip_shortcodes($post->post_content)), 80).'</li>';

			}	

		$return_html.= '</ul>';

	}

	if($echo)
	{
		echo $return_html;
	}
	else
	{
		return $return_html;
	}
}

function pong_cat_posts($cat_id = '', $items = 5, $echo = TRUE)
{
	$return_html = '';
	$posts = get_posts('numberposts='.$items.'&order=DESC&orderby=date&category='.$cat_id);
	$title = get_cat_name($cat_id);

	if(!empty($posts))
	{

		$return_html.= '<h4 class="widgettitle">'.$title.'</h4><hr/>';
		$return_html.= '<ul class="posts">';

			foreach($posts as $post)
			{
				$image_thumb = get_post_meta($post->ID, 'blog_thumb_image_url', true);
				$return_html.= '<li><img src="'.get_bloginfo( 'stylesheet_directory' ).'/timthumb.php?src='.$image_thumb.'&h=60&w=60&zc=1" alt="" /> <a href="'.get_permalink($post->ID).'">';
				$return_html.= '<h6>'.pong_substr($post->post_title, 50).'</h6></a><a href="'.gen_permalink(get_permalink($post->ID), 'quick_view=1').'" class="quick_view" title="Quick View"><img src="'.get_bloginfo( 'stylesheet_directory' ).'/images/icon_quick_view.png" style="width:16px" class="mid_align"/></a>'.get_the_date().'<br/><br/>';
				$return_html.= pong_substr(strip_tags(strip_shortcodes($post->post_content)), 80).'</li>';

			}	

		$return_html.= '</ul>';

	}

	if($echo)
	{
		echo $return_html;
	}
	else
	{
		return $return_html;
	}
}