Podcasts Connect art issue

Yes: that was quick! I can't see anything wrong with the image - it's .jpg, not .png as you stated but that's fine, and it meets all the parameters and passed a validation check. I don't see head requests (the ability for a client to request just the metadata without having to download the entire file) as being an issue as you're hosted by Buzzsprout who I would assume are across that, particularly as that checks out for the actual media file (I can't test the image).

So I don't see a problem. Wait a bit and try again: it might just be a glitch - It wouldn't be the first. If it still doesn't work, try taking it up with Buzzsprout. Failing that, you could try Podcasts Support though they won't help with something they perceive as being your fault: you'd have to convince them that the image is in fact OK.

The head requests issue is part of a standard response, and it's probably not the issue. If you post the URL of your feed I'll have a look at the image (it'll be tomorrow unless you're very quick).