Muzy.com is a photobox collage site that is really cool. Actually students do not even have to have an account to use the site but would need to be able to screenshot their work and then they could use it wherever they needed in projects or just alone for an assignments that revolves around pictures. Without even setting up an account I created two pictures. The first one using the templates under photobox where you just select pictures and edit simple text.
The next example is using photo collage where you create your own collage and text.
I can see some many quick uses of this tool in the classroom to get students to think in terms of pictures and outside of the box. It is super simple and easy to use. Give it a try!
I loved picnik and miss it dearly and although nothing seems to have replaced it for me the best site I have found that is simple to use is picmonkey. I used it for my e-Christmas Cards this year and was pretty happy with the site overall and I used a few before I decided this was my favorite and easiest to use. This can be used without an account so students could decorate pics before using in something like Muzy.