Png images with black background

Looking at the referred post and taking a look at the class-Category-Images-II.php file I think that the problem resides in how the plugin generates the thumbnail image:

protected function save_thumb( $original_file, $thumb_file )
	{
		// Scale the image.
		list( $w, $h, $format ) = getimagesize( $original_file );
		$max_side = $this->get_max_side();
		$xratio = $max_side / $w;
		$yratio = $max_side / $h;
		$ratio = min( $xratio, $yratio );
		$targetw = (int) $w * $ratio;
		$targeth = (int) $h * $ratio;

		$src_gd = $this->image_create_from_file( $original_file );
		assert( $src_gd );
		$target_gd = imagecreatetruecolor( $targetw, $targeth );
		imagecopyresampled ( $target_gd, $src_gd, 0, 0, 0, 0, $targetw, $targeth, $w, $h );
		// create the initial copy from the original file
		// also overwrite the filename (in case the extension isn't accurate)
		$success = false;
		if ( $format == IMAGETYPE_GIF ) {
			$success = imagegif( $target_gd, $thumb_file );
		} elseif ( $format == IMAGETYPE_JPEG ) {
			$success = imagejpeg( $target_gd, $thumb_file, 90 );
		} elseif ( $format == IMAGETYPE_PNG ) {
			$success = imagepng( $target_gd, $thumb_file );
		} else {
			wp_die( __( 'Unknown image type. Please upload a JPEG, GIF or PNG.', 'category-images-ii' ) );
		}
		if ( ! $success )
			wp_die( __( 'Image resizing failed.', 'category-images-ii' ) );
	}

Unfortunately, I’m not a code master to try to fix it. Any solve?

ps: sorry for my english

Thanks madjax! I will take a look.