mbecher

This plugin will extract the large version of an image attached 
to a post and put it in front of the content in the RSS feed. 
This might be useful if you have an image (and only a single one) 
attached to a custom field and it won’t display in the feed otherwise.

RSS Custom Fields Images

Puts large sized image attached to posts in front of content in rss feeds. Useful for images in custom fields.

ajayver

This plugin gives you the ability to change the appearance of your RSS feed. You can easily add images from custom fields, read more link, category list, tag list and any custom code before or after the feed items. Also, the plugin allows you to delay feed publication for several minutes. It is helpful, when you made some mistake and you will have time to fix it before the post went to RSS and all other services like facebook or twitter.

RSS Manager

This plugin gives you the ability to change the appearance of your RSS feed.

RSS Custom Fields Images vs RSS Manager