Yeah I think its fine to have covers for downloadable games. Gives you something tangible to say you've got the game :p I printed a Sonic 4 cover off before it came out :)
Edit: Made a new one. Been a while. Twilight Princess tiem!
http://www.mediafire.com/i/?cj3qle1q6jjozdz
Yeah, the image is flipped making Link a 'righty' but he couldn't very well sit on the back cover. :p
And Metroid Other M:
http://www.mediafire.com/i/?3m58ml28xp58wxw
And Mario Sports Mix:
http://www.mediafire.com/i/?a5qbakr6eywqa3g
Aaaand DKCR:
http://www.mediafire.com/i/?gs60baw4lcdlnoo