Grab a thumbnail for post... If no featured image... Grab the first image in the post content.. Or generate a thumbnail based on youtube video if that is included instead of an image...

grab thumb url.php

<?php

function rw_thumb_url($text, $size){
	global $post;
	$imageurl="";
	
	// Check to see which image is set as "Featured Image"
	$featuredimg = get_post_thumbnail_id($post->ID);

	// Get source for featured image
	$img_src = wp_get_attachment_image_src($featuredimg, $size);

	// Set $imageurl to Featured Image
	$imageurl=$img_src[0];
	
	// If there is no "Featured Image" set, move on and get the first image attached to the post
	if (!$imageurl) {
		// Extract the thumbnail from the first attached imaged
		$allimages =&get_children('post_type=attachment&post_mime_type=image&post_parent=' . $post->ID );
		
		foreach ($allimages as $img){
			$img_src = wp_get_attachment_image_src($img->ID, $size);
			break;
		}
		// Set $imageurl to first attached image
		$imageurl=$img_src[0];
	}
	
	// If there is no image attached to the post, look for anything that looks like an image and get that
	if (!$imageurl) {
		preg_match('/<\s*img [^\>]*src\s*=\s*[\""\']?([^\""\'>]*)/i' ,  $text, $matches);
		$imageurl=$matches[1];
	}
	
	// If there's no image attached or inserted in the post, look for a YouTube video
	if (!$imageurl){
		preg_match("/([a-zA-Z0-9\-\_]+\.|)youtube\.com\/watch(\?v\=|\/v\/)([a-zA-Z0-9\-\_]{11})([^<\s]*)/", $text, $matches2);
		$youtubeurl = $matches2[0];
		$videokey = $matches2[3];
		if (!$youtubeurl) {
			preg_match("/([a-zA-Z0-9\-\_]+\.|)youtu\.be\/([a-zA-Z0-9\-\_]{11})([^<\s]*)/", $text, $matches2);
			$youtubeurl = $matches2[0];
			$videokey = $matches2[2];
		}
		if ($youtubeurl)
			$imageurl = "http://i.ytimg.com/vi/{$videokey}/0.jpg";
	}
	
	// Spit out the image path
	return $imageurl;
}