Can you mail a 4x6 Photo as a Postcard?

My friend uses photos as postcards all the time! It's the most personalized way you can say "hey check this out!" She just draws a line down the back, writes her message on the left, address on the right, and sticks a post-card stamp on it! They arrive a little scratched, but if your purpose isn't to send an award-winning photo to be framed and hung on the wall, then it usually isn't a problem.