I want to create a new forum for it and maybe a linked gallery, I think that "patchworks" could be fine.
An user make a picture and then leave it to the other users, who wants to partecipate has to use this picture to complete the scene, the first picture could be even in a PSD format with the alpha channel or and/or with a uniform background color (maybe white) so that it's easy to cut it perfectly and paste it in a new context made by the participants
. The option for the user who donate the picture could be to give a limited permission to share the picture or not, for example I'll give the permission to use my picture for a work that will be published on Posetteforever only in the dedicated gallery, not in other places, but an user can (and have to) choose where the final picture is supposed to be published. I think that we can have a lot of fun with this one, I had the idea after posting my picture in the MBOW gallery, maybe it could have been nice to have another user complete it and see the difference 
