Not exactly sure if this is the right place to go, but..

If a image is set as wallpaper from any place on Nautilus (on the home directory, for example), the image will be:

1 - Copied to ~/Images/Wallpapers
2 - Set as Wallpaper

This is ok, consider the possibility of the original file become deleted and the wallpaper changes to "null", but...

- If the file already exists, or at least exists a file with the same name and extension, it would be substituted, instead of consider the two files as the same and just.. use it..
- If someone try to use a wallpaper from there, the image will be copied and the copy will be set

Why not change the behavior to:
1 - Check if the file exists
    if exists check the both files and see if they are the same, if they are, use the one in ./Wallpapers, if not, check if substitute
    
    if the file don't exist, create and use it from there

Also, if the file is the Wallpaper, it just delete. Shouldn't be checked if the Wallpaper should be changed or not before delete?