Take a look at wallpaper apps on Google Play, I think they make their money through in app advertising or possibly in app purchases though I have never used them.
If you Just Google cell phone wallpaper on the web you will get a list of sites check them out and see how they market their wallpaper. A lot are listed as free so they to may be making most of their money off of the advertising on there web site. You could try to establish a similar type site.
Perhaps the easiest thing to do would be to contact those site to see if they are looking for new inventory and sell to them. Unlike the first two methods, you wouldn't have to acquire an additional skill set or pay someone for assistance.
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