The <image> tag that is above the PowerPress comment line "<!-- podcast_generator="Blubrry PowerPress/6.0.5" mode="advanced" -->" is from another source. It could be from a plugin (you ruled that out, but perhaps you didn't if your server or a WP caching plugin is caching files), it also could come from a theme. Vanilla WordPress with the twenty fifteen theme and no other plugins should not be doing this.
Your hosting is using nginx web server so it is completely possible that there is a server based cache. If your using an aggressive WP caching plugin, there may also be files cached that are handled before PHP is executed. Key to debugging when turning off plugins is to clear your caches every time.
Here is a trick you can do during your debugging. Add a question mark and something random (random every time) to the feed URL.
A hint for me is that the first image tag includes an image that is 32x32, which hints at being a favicon setting of some sort. Also it is using this image (
http://thecrummyshow.com/wp-content/uploads/2015/09/cropped-site-logo-wp-32x32.png) which is not your podcast artwork (
http://thecrummyshow.com/wp-content/uploads/2015/09/itunes-art.jpg), so you may want to trace down where you uploaded this image.
Last thought, this is your RSS image, you do want to get this fixed but it will not effect iTunes, as that's a different tag all together.