RSS Custom Fields Images

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.

Extract the zip file and just drop the contents in the
wp-content/plugins/ directory of your WordPress installation
and then activate the Plugin from Plugins page. See if you
like, what it does to your RSS feed. There is no options page.