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.

rgubby

If you’ve ever wanted to use WordPress feeds to deliver content into other areas, and want to do it via custom fields, you’ll know that WordPress feeds don’t display custom fields.
With this plugin, you can show all your custom fields in your feed so you can pull data out of WordPress and use it with other systems.
Custom fields are available inside a element in each .

RSS Custom Fields

Allow your RSS feed to display custom tags

RSS Custom Fields Images vs RSS Custom Fields