I never had any luck myself finding a Mac collage maker. I have a powerbook G4 though and had Virtual PC on it running Windows 2000 and found a Windows based collage maker called "Collage Maker" lol that worked great. It's from galleriasoftware.com and they let you try it before you buy it. Of course if you don't have windows on your Mac this is pretty useless information for you now. lol
Your best bet is to purchase Photoshop Elements (make sure the version you purchase is MAC compatible) than start a "New Project." Set the adjustments of the document to the size you need than make the collage out of copied background layers. If you want to go this route I can help you. Just message me on the forums.
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