So I originally found the idea of character collages at Literary Rambles which led me to Seeing Creative's post on it, which finally led to the original source on WriteOnCon. I'll let Tera Lynn Childs explain it:
I thought it was a cool idea, but I'm a poor college student, I don't spend money on magazines, and I don't even have a printer of my own. So while I thought it was so cool and I wanted to do it, I didn't. Until today. You see, my semester is ending and our dorm is all packing up and getting ready to leave, so people are putting out things they don't want anymore and anyone can take. Among them: a massive stack of Entertainment Weekly magazines.
I grabbed them and started up two collages for different characters. I just made them today, so we'll see how they work for my writing process, but as for character building, it helped. I'd been struggling with a way to describe one of my characters, and then in the magazine they showed an old picture of Gregory Peck, and said he played the "quietly courageous Atticus Finch." And then I realized that's how I wanted my character. Quietly courageous.
So I don't have a scanner, or else I'd totally post my collages. But if you have a stack of magazines and time, I suggest this exercise!