I've recently discovered a free iphone app called dubble. You upload a photo & then it combines with someone else's photo (from anywhere around the world) to make some really fun double exposed pictures.
These are a few that I've created, I love the way they look & it's really fun (& a little bit addictive).
The collaborations are random and the photos below are my photos combined with these dubble app users :- conorlocky, g0sp0di, secutor, thankgravity, yenwong, dukeofbkk, freezeek, maulaali
Why don't you give it a go?