HBO just pointed out that there's a new website created just to supply Halo based wallpapers for many devices. While this seems  a fairly simple idea, its weird that no one has come forth and done this already. I have had a look threw and already picked one for my Giant Bomb profile which is up now, 
as well as a few for my Desktop background. The pickings are slim at the moment, and it seems to be Halo 3 / Reach orientated but I am sure this site will take off.   
 
If your wanting to take a look, or are in need of a fresh lick of paint/halo on your device, id suggest you check it out.  
 
http://halodesktops.com/