browse your picture repository and select an image choosing  "Set as wallpaper" from the context menu

If doesn't exist, the folder $HOME/Pictures/Wallpapers is created and a copy of original image is placed in that directory. I guess, the copied image  is used as wallpaper.. this could be acceptable even if a sort of softlink could be used instead of  generate a 2nd copy of the original image..

Anyway, if you go into the $HOME/Pictures/Wallpapers, choose the image and select again "Set as wallpaper" another  copy of the same image is created in the folder..

if you choose the new copy of image,  select  "Set as wallpaper" another copy of the copy image is created in the folder..

and so on..

I think that  should be managed in a better way an existing image already present in the $HOME/Pictures/Wallpapers avoiding the creation of multiple copies of the same image...

Cheers
Andrea