Bing Desktop local image folder

Resetting permissions on the file and overwriting will not work, as shell32.dll has code which compares the image with a SHA-256 hash. The only option other than tampering with the files yourself in a hex editor is using Stardock MyColors, though you must use an entire theme.

FWIW, I found this answer trying to fix this same issue. It seems that Microsoft broke or otherwise disabled Bing Desktop in favor of Bing Wallpaper (on Windows 7 at least) for wallpaper rotation. The steps above didn't help me, but I uninstalled Bing Desktop and install Bing Wallpaper (https://www.microsoft.com/en-us/bing/bing-wallpaper) and now I have my backgrounds again.