You remind me of something different but related. I noticed in Windows 98 that it would store a different setting for BMP wallpapers. And BMP wallpapers would also load in faster. So when you first set a BMP wallpaper, and then change it to a JPEG, it would remembrr both of threm. And it will still briefly display the BMP one on startup before it loads the JPEG one.
Oh this unlocked a memory of the hassle I went through trying to get rid of jpeg artifacts on my wallpaper on windows 7. It really pissed me off.
You remind me of something different but related. I noticed in Windows 98 that it would store a different setting for BMP wallpapers. And BMP wallpapers would also load in faster. So when you first set a BMP wallpaper, and then change it to a JPEG, it would remembrr both of threm. And it will still briefly display the BMP one on startup before it loads the JPEG one.