Plugin URL: http://wordpress.org/extend/plugins/sb-rss-feed-plus/
Add post thumnail to each post's excerpt and full text (if enabled in WordPress configuration). Image will be added just before text of each post in RSS feed. Post thumbnail is also added as media:content and enclosure tag to RSS feed.
You can add server signature just after the full text content of post to each RSS post.
Very simple implementation of advertisement to each RSS feed post item. Ad is a simple clickable image (stretched to 90% of width - maximum of 700px).