my brother and bhabhi's photographer provided a 16" x 20" photograph with a large matt and frame. the purpose of the matt was for guests to leave their congratulatories on - it basically replaced the traditional guestbook. it's nice because they're faced with everyone's best wishes every day since it hangs in their bedroom. the biggest issue was the pens i bought for people to write with. i got the silver and gold liquid pens. since the picture was sitting on an easel, the pen would slowly stop working as you write. then someone shook the pen and the ink went all over their picture :S eek.. the photographer was good though; he replaced the picture for free!
anyway, going on the same idea without the pen issues, here's another idea for a "picture frame guest book": it's an engravable frame! it's gorgeous and something you wouldn't be embarrassed to hang. (unless someone were to write an inappropriate message or draw a lewd picture :S)