Recommend free photo collage building software, please

We combine our photos into large collages, A2 or A1 size. In the past we’ve done this by cutting up the photos and physically pasting them, but we can’t do that for the next lot. We’ve run out of printed photos, for a start, and I don’t fancy printing a whole load just to cut them up!

It looks like there are a whole bunch of software options to do this. No idea how to choose between them, so I thought I’d ask for personal recommendations. Have you used anything? What would you recommend?

The key requirement is that I need to able to cut in a bit of a photo into the finished work, not just include the whole thing. We use Picasa if that’s important, so I could do edits through that I guess. I saw a mention of Picasa’s own collaging tool, but I haven’t looked into that at all, so if you’re familiar with that that would be interesting. The plan is to hand the finished collage file to a photo print place to print the right size.

Thanks in advance, if this turns out to be easy enough to do I might finally get some photos of my youngest child up on the walls… poor neglected second child!