(sorry about the links-i cant figure out how to post a picture)
Here's how to post a thumbnail --
Find the picture(s) you want to post, check in the little box next to it/them, and go to the bottom of the page and click on the box at the far right that says 'generate HTML and IMG code'. Then you'll go to a page with a bunch of different boxes of texts and you want to copy and paste the third one that says 'IMG clickable thumbnails for message boards'. Just paste the code into your message here.
Hope that helped!