Hope this is in the correct place , where do people that own wallpaper sites get there wallpapers from , do they get them from other wallpaper sites? or buy them?
there is the lots of way to get logo but the batter to you that make is design or hirer the people who snap the wall paper
It depends on the copywrite of the wallpapers basically. I can help you out in finding few sources for good wallpapers. For quality and more info check my wallpaper blog, fabulous wallpapers. (Link in my Signature.)
On most of the sites users contribute. You find 3-4 people that will put like 20 wallpapers each, and then you grow up your community. Also, you can make some by yourself. Photoshop and go go
I would suggest having a script installed that allows you to have a gallery and allows people to upload wallpapers. that is the best way to grow your site.