There's a thread on Photo Posting, but essentially you have to have the photo somewhere on the web. Like a flickR, PhotoBucket, etc. Then you link to BE by copying the web address (the "Properties" of the photo). Then paste into your reply. Syntax is:
{img}protertiesyoucopied{/img}
BUT!!! Notice I used the "face like" parenthesis. You have to actually use the square ones on the same keys. The square are so special to webpages that I can't post them for you as an example.