Include image in RSS feed Views display
If the image is a CCK field, make sure the correct formatter has been selected for the image field and it has not been excluded from the RSS display at admin/content/node-type/YOUR-TYPE/display/rss.

If it's an attached image, check that it is displayed as part of teaser (or full body) and that Feed content is set to teaser (or Full text) at admin/content/rss-publishing.