Currently, wallpapers are accessed from their original paths. This can cause the wallpaper to disappear and display a black background when the file at the path disappears, for any of the following plausible reasons:
- The user was using a wallpaper provided by a distro package that's removed in an update
- The wallpaper was located on an external disk that is unplugged

It might make sense to copy the wallpaper to an on-disk cache location to prevent the background from turning black when one of these conditions is encountered.