Your WordPress theme may be adding an additional image tag.
Try this:
1.) Log in to your WordPress site and hover over the PowerPress link in the left-hand sidebar. Click the Settings link in the popup menu. Switch to Advanced mode if you're not already there
2.) Click the Feeds tab. Check the box next to "Do not allow other plugins to modify podcast feeds" and click the Save Changes button
See if that removes the unwanted image tag from the feed.