Weird background in Windows 10

Ever since I switched to a custom background, I've been experiencing some problems.

My Windows 10 Background Is supposed to look like:



But if a fullscreen program runs, a UAC Windows pops up, etc, It will look like:





Is there a way to fix this?
and why is this happening?

Very strange. Never seen this before and i tried hundreds. This happens with other backgrounds too? Don't know how to fix this.