Currently Being ModeratedFeb 12, 2013 12:34 AM (in response to cabassi)
I figured it out. A simple fix. When exporting a podcast to disk, there's an option to "Set artwork to recommended size for podcasts (300 x 300 pixels) when exporting." I had that checked, so I unchecked it and reexported.
Currently Being ModeratedFeb 12, 2013 1:08 AM (in response to cabassi)
Well found. The recommended size used to be 300 x 300 and obviously Apple haven't brought GarageBand up to date.
However you may find that the iTunes Store doesn't update the image if the file name hasn't changed, so you may want to see if you can give the image a new filename and reupload.