Originally Posted by taylormemer
It's because you didn't resize the image to something the site is willing to handle, such as the default album art display size: 200x200. That image you tried was indeed jpg, but the source of the issue came from the fact that it was 1600x1600 (over 1Mb) which is just unnecessary.
tl;dr your image's file size and/or dimensions caused the site to crap out - fixing now.
EDIT: despite all that, I'm not sure why the site rejected the art (unless it needed to be approved by us first, which is possible)