I discovered the Felt iPhone app which made it so easy. I could either use their images or my own. They are located in Colorado, so not only was it mailed from the U.S. to my friends in California and Washington, but I was helping the U.S. post office. They actually put a stamp on the envelope and addressed it for me.
Plus who doesn't love to get a snail mail card these days? An online card is just not the same as going to your mailbox and getting a surprise.