Website where I can create a free montage/collage 4 prtflio?

I have a whole lot of sketches that I don’t want to put in my portfolio individually, but I would like to to create a few montages/collages of 50 or so various images…something very simple, all images the same size in something like a 10x5 or 6x8 format.
I have a found a few apps which PC users can download to create what I’m looking for, but I have a mac so they’re no use to me. I feel like this is such a basic thing I should be able to do it in a million different places and there may very well be a whole lot of sites out there that can do just this, I’ve just yet to find one.
any help would be much appreciated (asap as i’m hoping to get my app in the mail by mid-week).

thanks in advance.