there's some mighty good info in this thread
I recall, from my days taking creative writing classes, that you can also mail yourself your work, and keeping the envelope intact (not opening it) will prove ownership of your material should a question ever arise as to whether the material is yours or not; the post mark date is verification of when the material was created. That obviously works better with documents than photography, though, and that was quite a few years ago, before many of the online options were available.