One can use Word
My daughter found a way to make collages using MS Word (if you have it, of course). Cut and paste the pic into a Word Doc. Modify it to be "behind text". As such, you can move it and it will stay wherever on the page you put it. Load more pics. You can crop them, or make them oval, or any free shape with the format picture tools (lasso, etc). Then use the "move to front" command to overlap in the manner you wish the pics to be. I am also looking for a non-MS means of doing this, but honestly, it works well. The only issue I have had is using multi-MB size pics. Reduce the resolution of the pic (in iPhoto or by other means) before moving it to Word.