Hi There,
We have already developed an app similar to your requirement.
It downloads wallpaper from server and present to user.
User can save and set wallpaper.
You can check this app here -
http://www.amazon.com/Gkart-Technologies-Pvt-Ltd-Bollywood/dp/B009L8A3IQ/ref=sr_1_4?ie=UTF8&qid=1392715966&sr=8-4&keywords=gkart
If you need demo apk, We can send you right away.
Thanks,
Gkart Tech.