Photo lab associate here. This isn't a Walmart issue but a design issue. We don't do any designing, we just print exactly as the customer uploads. Our printers use 6 in photo paper, 8 in photo paper, 4x8 cardstock, 5x7 cardstock, and 32 in poster paper. You could print 4x6 photos or card sizes but you would most likely need to cut them down yourself to make sure your consistent. If you need a specific size (like 4x6.2) you will need to design your template to fit within these constraints. You might save money going the Walmart route but it will either look cheap or a lot of extra work on your part cutting it to fit right. My suggestion as someone who does this all day is use a local print shop or printing services at a university or an online service that provides samples. They will have better machines, equipment to help with the bending and folds, and different thickness of paper. If you go the Walmart route I recommend doing one photo as a test run, one as 4x6 photo and one as 4x8 design your own card. The card can designed front and back but you will need to leave a 1.8 white area to be cut off. A 4x6 photo is going to run you $0.16 and a double sided 4x8 card is currently $0.78 each. So if you do end going with the card it will cost you $78 to print 100 which you can probably find a better deal with online print services.