I am having a similar experience with custom template backgrounds for one of my custom sheets. The problem is not persistent across all images, but many of the template images are affected in my case. This started on Friday and has not resolved itself. What I did to mitigate the problem for our game last night, was move the images to a different image hosting site (shutterstock instead of imgur) and it worked. It was a pain, as I had to revise the background url for 60+ templates, but it fixed the problem. Anyway, I saw your post and wanted to let you know that you're not alone. If it is something on Roll20's end, hopefully someone will see this and fix it. I wasn't sure it was a Roll20 problem, as the symptoms pointed more to the image hosting site than Roll20, but your scenario has me second guessing that assessment. Good luck and hopefully you get it working.