Sebastien, some wallpaper entries in the 'Picture Folder' (System Settings → User Interface) set the 'picture-uri' key to 'file:///home/<user>/Pictures/<wallpaper_filename>', which is readable and will be used by unity-greeter.
So either that shouldn't happen, or the other wallpapers should work too.
Sebastien, some wallpaper entries in the 'Picture Folder' (System Settings → User Interface) set the 'picture-uri' key to 'file:/ //home/ <user>/ Pictures/ <wallpaper_ filename> ', which is readable and will be used by unity-greeter.
So either that shouldn't happen, or the other wallpapers should work too.