Thank-you for your response! I'm confused by it, however. I have looked over my feed many times, and I don't think the image is given as http://biochemrevealed.org/logo.jpg. The image should be at http://www.biochemrevealed.org/Biochem_revealed_logo.jpg, which does go to a valid address. Can you tell me where in the feed it is pointing to logo.jpg?
As an alternative to fixing the rss feed (though I also really want to understand what is going wrong with it), I have also taken my logo and uploaded to the server as logo.jpg, so now biochemrevealed.org/logo.jpg is also a valid address containing the image. Should that fix the problem?
Certainly the feed now gives http://www.biochemrevealed.org/Biochem_revealed_logo.jpg and there is a logo at that address. It's also now appearing on the Store page.
I would have copied the other URL from the feed, but of course it's not possible to check whether an error got in somewhere, or whether the URL in the feed has mysteriously changed since I looked at it. At any rate everything now appears to be OK.