...same as for any other site. It must comply with the guidelines and offer UNIQUE and COMPELLING content.
You need too description for every wallpaper, if you want traffic and if you want rank well. Website only with pictures (wallpapers) can't rank well. I suggest you to post some articles (unique) about how to create wallpaper, how to edit wallpaper or something like this... Then you will get approve. But don't expect approve, if you have less than 50 traffic per day.
Just apply with your site and see how Google respond. most likely rejected with message: "site does not comply with google's policy"