Wordpress function to get a thumbnail image filename from a different sized thumbnail or the the source

jimcode_get_thumnail

	// utility for getting a thumbnail image from a different sized thumbnail image
	// or the original. Assumes the size requested is auto generated
	// eg. ..../2012/05/TSStAlbans-h1091-1024x750.jpg
	//     to../2012/05/TSStAlbans-h1091-300x200.jpg
	//
	// Or. ..../2012/05/TSStAlbans-h1091.jpg (source image)
	//     to../2012/05/TSStAlbans-h1091-300x200.jpg
	//
	// Add standard image sizes using add_image_size function and rebuild
	// thumbnails when needed using AJAX Thumbnail Rebuild plugin.
	//
	// See:
	// 		 http://codex.wordpress.org/Function_Reference/add_image_size
	// 		 http://wordpress.org/extend/plugins/ajax-thumbnail-rebuild/
	function jimcode_get_thumnail($existing_filename, $width, $height) {
		
		// do a clever regex replace
		$filename =  preg_replace('/(-\d+x\d+)?\.(\w{3,4})$/', '-'.$width.'x'.$height.'.\\2',$existing_filename);
		if ($filename == $existing_filename) {
			// we didn't change anything - so check if we matched the same dimensions
			if (preg_match('/-\d+x\d+\.(\w{3,4})$/', $existing_filename) == 0) {
				$filename =  preg_replace('/\.(\w{3,4})$/', '-'.$width.'x'.$height.'.\\1',$existing_filename);		
			}
		}

		// check if that actually exists and if not just return without any size	
	
		// get the url without the domain
		$path = str_replace(get_option('home'), "", $filename);
		$path = ABSPATH.$path;
		$path = str_replace('/', DIRECTORY_SEPARATOR, $path);
		$path = str_replace(DIRECTORY_SEPARATOR.DIRECTORY_SEPARATOR, DIRECTORY_SEPARATOR, $path);
		if (file_exists($path)) {
			return $filename;
		} else {
			return preg_replace('/(-\d+x\d+)?\.(\w{3,4})$/', '.\\2',$existing_filename);;
		}
		
		return $filename;
	}