Having postage made using your card images is an easy and fun thing to do. I have had several sheets of 25 made over the last year or so.
You do need to place a minimum order, you cannot order one stamp. I always order sheets of 25, and am presently in the process of picking out destination US stamps to do.
For Canadian Picture Stamps you can head to Picture Postage to design your order.
In the USA you can find picture postage at Create Mail and Stamps and the program you want is just a bit down the page called PictureITPostage.
Above are the stamp sets that I have made so far, with a picture of the card that was used for one of the sets, to give you an idea how it all looks.
I hope you will give it a try, as cardmakers we all use a lot of stamps.
LONG LIVE SNAIL MAIL