I have tried multiple times with different browsers and different photos (even changing the filename), but the background images cannot be uploaded.

Sorry you're having issues. We have not been able to replicate the problem on our end. Have you tried refreshing the cache? That will probably resolve the problem. Please keep us posted!

Unreliable website, I would say. There's always a glitch that makes it impossible to use it. One day it's the icons, you can't set them. Then, the website is unavailable. There's always something that's out of order. Several times, I've just given up and left. I recommend anyone who uses this broken thing to find something else that works.