Maybe it's more common with the Tor think on Full? No idea tbh
If going through Tor, it might be that Tor is too busy at that moment.
I wonder if it could be an idea to have info on the notification? The worry is someone tries, fails, thinks it's because the image is roo large, reduces image /size quality, tried again, fails again (and again). And think ah you can only post extremely tiny image sizes? (That's what I thought lol)