I just answered this question How can area be a vector? and included a couple of images. I think especially for novices a picture of what you just wrote can go a long way to explain the concept. I "stole" those from wikipedia. Is there a way to scale them down a bit from the original size, since they turned out a little too big.
I don't think markdown provides any help, but perhaps including the images with html tags and size attributes would do the trick.
Otherwise you could copy the images to your machine, scale them with your favorite graphics tools and upload them from there.