Featured Images in RSS is a WordPress plugin by Rob Marlbrough that adds features images in RSS feeds, with easy size and positioning options.
Works great with Feedburner, Chimpfeedr, and Mailchimp’s RSS-to-Email campaigns, with the featured images added to the left or centered above the text, or to the left of the text, and with thumbnail, medium, large, or full sizes. It’s WordPress Multisite compatible, simply Network Activate to add featured images to all site’s RSS feeds. It defaults to thumbnail sizes with left above text positioning.
Download: http://wordpress.org/plugins/featured-images-for-rss-feeds/