If you already have them saved as card images, you'll need to upload them to a website that can host image files. I usually use tinypic.com because it's fast and simple (and the links are short), however if you want to be able to organize/sort your card images or keep good track of them you'd want to use something like photobucket.com as it has an account-based system and you can create collections or folders.
Once you've uploaded it just copy the "direct link" for the image and then in a post on the boards you put that link in between img tags (just remove the periods from the code below):
[.img]direct link here[./img]
You'll end up with something like this:
