Here are the instructions. (I guess that's a specific How To thread I should create in the FAQ board.)
When you are in the "Post Reply" box, there's a little icon that looks like a framed picture in the second row of icons. (It's very small and hard to make out.) It's the fourth icon in from the left. Mashing that button brings up these tags [ img ][ /img ]. (Without the spaces. I had to put the spaces in to make them show up.)
You upload your images to a photosharing site like Photobucket or Imageshack, and then get the image's URL from there. (Photobucket will give you the URL with the [ IMG] tags already already attached for copying and pasting into a post.)
Once you have your URL copied, you paste it in between the two [ img][ /img] tags.
Or, alternatively, paste your image's URL into the reply box, highlight it, and then and then mash the image icon. That'll place the [ img] tags on either side of your URL for you.