I use my word processing program...under labels. But you can just take a sheet of 8.5 x 11 cardstock and cut it fourths. Each piece is a post card legal size. I put the entire sheet (page 1) in my printer and use my computer graphics to print something cute or noteworthy on each one - individually or as a group. I also type three lines on the reverse side (page 2) for writing the address. This is not necessary, but let's people I am gifting with the post cards know what they are to be used for. Then just print and cut apart - usually 4.25 x 5.50. Put proper postage on 'em and mail.
The Post Office has no rules that I know of for what you send through the mail. I used to work outgoing mail for the Post Office back in the 90s in a college town. You wouldn't believe what we saw coming through to be mailed. One time it was a bra just before St Patrick's day. It had the address on one cup and the appropriate postage stapled on the other cup along with the message "Erin go Braugh! Also I remember a Hershey bar wrapper coming through acting as an envelope. (04/03/2009)
I have used photographs, the front cut from a greeting card, or a picture from a magazine glued onto cardboard, etc. Since the post office uses machines, it may be a good idea not to glue on any decoration that could get caught on the machine. Also, keep it close to postcard size since a larger size could change the postage. I always put a line down the center of the card - message on the left, address and stamp on the right. (04/18/2009)
Try Postcards.com You can upload your own photos and customize it on their website. They print them on high quality paper at the correct size so you don't have to worry about the little details. Just make it your own and send them out to your friends, they can even send them out for you. Check them out, it's a great way to send holiday cards, invitations, or greetings! (10/08/2009)