Add a namespace attribute to your RSS feed.
<rss version="2.0" xmlns:pb="http://snaplog.com/backend/PhotoBlog.html">
Add a thumb element to the rss/channel/item nodes.
<pb:thumb height="105" width="140">
http://tastesgood.org/2006/03/25/135417t_Random_NYC_078.jpg</pb:thumb>