First you need to have the cards you want to caption. That's easy enough. Just go to the card section of the forum and pick any card you want. Even if there isn't enough space on it, don't fret, I've got that covered. If you look just below these steps you will see a dark box that looks similar to the boxes found on cards. Just save that to your computer as well.
Now that you have your cards, right click it and save it to your computer. Now you can open it up in your own graphic editing tool or you can simply use this free online program:
http://www.pixlr.com/editor/ As soon as you go to that link, you will have an option to open an image from your computer. If your chosen cards have too many words on it to easily add to it you will need to open that dark box you saved earlier along with your cards.
Now you just select the typing tool(It looks like a big A) and type on the card. It's best to make a new layer first.
For those of you that have to use the dark box, use the selector tool(A square made up of dashes) and select everything in the dark box window. Go up to edit, then cut, then change over to your card window and paste it on a new layer. Now you can extend the canvas(Image>Canvas Size), click on the top middle box and in the 'Height' window add 64 to that number. Now the dark box will fit comfortably under that card and you'll have room for filler text.(Just think that if this were to ever get in the game, it could be an extra pop up option or the like.)
These instructions are basically the same for most graphic editing programs. If you need any help feel free to ask!
Once your image is all fine and done, just save it(File>Save>Select .png as the type of file and name it)
then upload it to any well respected image hoster. I might recommend
http://www.imgur.com/ PM your submission to me when you are done with your cards.