These are 2.5" x 3.5" works of art that are traded (never sold) like any other card (baseball, Pokemon, etc). I've seen these on Swap-Bot and finally looked into it and joined a few ATC swaps (ok, 10).
Here are some pictures of ones I've made:
"The Angel Underneath"
emossed stamped image of framed Victorian lady
"Saving For A Rainy Day"
I also have more made (series are Patriot, Polka Dot, etc) and have more planned. Swaps I'm in are monsters, Bible stories, my favorite animal, my partner's favorite animal, ABC (something that begins with those letters), and chicken/fowl. They are really fun to make and use minimal amounts of time, space, and supplies. I really enjoy it... I have always loved are and creating, but haven't done much since I've grown up. This is a great way to get that back without all the mess, time, and money!