Haha, now basically, open the atachment so it pops up larger, then click it, it should open a new window with the full size picture, alternatively with firefox you can just middle click the thumbnail and it will open a new tab. Then when you are on the new tab with nothing but the picture showing, you can simply copy it's URL and there is a button in the top of the post box that looks like a picture, you just click that and paste in the URL, or i personally just type out the code which is {img}img url pasted in middle{/img} but replace the wiggly brackets with the square ones.
I'm just playing with ya mate, bit of beer, bit of a big joint, you know how that ball rolls
straight off the lane and no strike for you sir