1
Open the Settings App, go to Personalization - Background, is that setting set to solid color, not Windows Spotlight, if so, set it to Picture, and set any picture as the background, then restart your PC and set that setting back to Solid Color to reset that registry key.
2
In the Settings App, go to Apps - Installed Apps, check there if the Bing Wallpaper app is installed, if so, uninstall it, that may be over-riding your settings for your Desktop background.