We first saw this layout for a photo collage at a friend's house, and I immediately wanted to do it for our guest bedroom! I love that together the frames make a substantial piece of art that fill the wall without being cluttered. I also like that we can change out the photos or use it in any house we move into. I purchased all of the frames at Michael's with 40% off coupons, so the total ended up being about $80. I didn't think that was bad considering a piece of art that large would have cost at least that much! The photos are of our immediate family who stay in the room when they visit. We used all sepia tone photos for a consistent look.
Although this layout could be done with any size frames, this is what I used:
Three-8x10" Photo frames with Mat (used vertically)
Three-5x7" Photo frames with Mat (used horizontally)
The key is to make sure the outer edges all line up, so the outside edges create a perfect rectangle. Since the frames are all different sizes the gaps between them will all be different, but if the outside lines up your eye won't notice!