There are some WordPress themes that add your site's favicon to the podcast RSS feed. I have no clue why they do this, but it happens.
1.) Log into 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.
Let me know when you've done that and I'll take another look at the feed and see if the extra image tag is still there.