Podcast artwork must be between 1400 x 1400 and 3000 x 3000 pixels, JPG or PNG, in RGB color space, and hosted on a server that allows HTTP head requests.

Having the exact issue, resized my picture to 1650x1650, and it still does not take it. Frustrating.......

In fact your image is 1400 x 1400 px with a file size of 201kB, which is good. However you have an alpha channel, and you should remove it.

How did you solve the problem? I am having the same one.

I use podbean for my host. After I uploaded the new artwork, I had to select it twice and save changes to get it to work. But, it finally did. Now it's caught up in the "pending review" queue.....