Help cant chance my windows wallpapers

[Alemm]

I installed 3 diferrent skins, and i though that i could change the background from them by right on the desktop, the problem was that when i choose the skins, those i didnt like i erased them, and the skins wallpaper are stocked there i've tried to change them but even if try the windows themes they wont change n if i try to send a wallpaper from the web it would say error you cant change your wallpaper.
what can i do to change the wallpaper?