Hi,
I understand your requirements
1) Take the wall Photo ( For this app will allow user to take a Photo or select Photo)
2) Then it allow user to select wallpapers from the list , it will sit on wall Photo.
I didn't understand one thing , you looking for Mac app or iPad app ?. In any case I can develop it for you . I have experience in both platform. You can see my apps in appstore . Please search for "Sheen Vempeny" in appstore.
Hope we can work togther.
Regards
Sheen