Podcast cover art must be at least 1400 X 1400 pixel JPG or PNG, in RGB color space, and hosted on a server that allows HTTP head requests.

It's not the image which is at fault - it's OK - but the fact that you are missing the 'iTunes declaration' at the top of the feed. The top two lines of your feed should read like this:

<?xml version="2.0" encoding="UTF-8"?>

<rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" version="2.0">

Your version is missing the second line (which is odd if Podbean have constructed the feed). Without it the Store and iTunes cannot read any tag beginning with 'itunes:', including 'itunes:image'; so you need to get that line into place.

I don't know enough about how Podbean creates feeds. If you can access the feed and edit it manually that should do it, but if Podbean generates the feed automatically when you add an episode it may over-write your editing. it might be worth contacting Podbean and asking for advice - it seems very odd that a podcasting company should provide a feed omitting this essential line: perhaps you have to specify that the feed is for iTunes.

I have no idea. How could I have, without being able to look at the feed?