You'll be fine. I've been participating in the Postcard Exchange for 10 years now (!) and have received all kinds of prints that have been sent directly in the mail (not in an envelope, although some people do that too). The most common postcard is the RC postcard you can get from Ilford - it has the traditional lines on the back that most postcards have for addresses, plus it's of thicker stock so is pretty durable in the mail. I've been using Fujibro paper (RC) which is much thinner, but I write all my details on white labels which helps make it more secure, and I've never had anyone tell me it didn't get to them alright. Some people do send fibre prints, alternative process prints (also, usually on fibre), and even instant prints, small as they are. As long as your photo is within the dimensions for what the post office will accept for the postcard rate, you can pretty much send anything, as long as you've got the address and postage stamp on the back.