Photoshop CS2 is an outdated version of PS that was made 100% free. It doesn't have all the bells and whistles of newer versions, but it has everything you need to make photo collages for CMON. It's what I use. LINK
if you want a free software, then gimp ( http://www.gimp.org/ ) is what I'd suggest.
It has most (if not all) the needed features of photoshop. (canvas size modification, copy-paste between images, color corrections, crop)