When uploading cover art recently, a few images out of the set invariably fail with “ Error uploading image”. Got it earlier today, figured it was just high-load time. But just happened again at ~2AM—and I doubt it’s a high-load time… (And since I forgot to hit create topic, again ~noon today)
Generally, hitting submit again gets them uploaded (and with a lot of images, sometimes repeat a few more times). But unfortunately they often get out of order.
I have been experiencing this same issue. It will happen if I am uploading just one image or multiple images, at what seems to be at random. I am using Google Chrome, beta version, which is kept updated.
For future reference, here you can check the status of the Cover Art Archives server. If you see a red line in the top-left graph, the server is experiencing difficulties. The only thing you can do in that case is wait until it is fixed.