I like to make my own greeting cards, thank yous, and invitations, and my favorite way to do them is in the 4 x 6 photo format. Everyone really likes them because they always put them on their fridge (my daughter's is full of them) or they can slip them in their photo albums. I then upload them to WalMart and they print them out for pennies.
Above is an example of one I made for my mom a couple years back. I chose a few very old photos and printed them out in grey scale. I even enclosed a photo of her own mother taken in the 20's. It's one of her favorites.