This isn't exactly for Cmas cards, but if you are looking to make prints of a photo or a collage of photos, you can't beat the 9 cent 4x6 prints at Snapfish. I took 6 different photos of my kids and made a collage and then added a line of text at the bottom. You can either have them shipped or you can pick them up at a Walgreens. Great to include in a regular Cmas card.
Wal-mart also does the collage prints and I think they are quite inexpensive too.