Send Images to RSS is a WordPress plugin that replaces images with an email friendly size image in RSS feeds. I like this for sending images from a WordPress gallery, for example--instead of sending thumbnails to the RSS readers, they get the full size images.
The plugin optionally adds a new email friendly image size to WordPress. Any large images uploaded to your site with this plugin activated will automatically have a new copy generated which is 560 pixels wide. If this image exists, it will be sent to your RSS feed, so we avoid the issue of overlarge images going out in email and your emails looking like monkey poo.