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.
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.