I've also done this type of thing for gifts. My parents have received these collages for holidays and a couple years ago, we did one for my husband for Father's Day. I found a bunch of pics of him and the kids fishing and let the kids decorate the mat with their own messages for him and fishing stickers.
Another fun way to use photos and pictures is to trim them down and glue them to a mat, using them to create a border for a larger picture.
These are a really fun and inexpensive way to display a ton of pictures all at once, without using every available inch of wall space :)