You must post the picture to an internet site and click "image" above your reply box. You will then be prompted to type the full URL for the picture.
Finding a place to upload the pic is, in my opinion, the toughest part. I had to ask someone on these forums if they would kindly upload my pic to their webspace, then emailed them my pic.
I've only posted one pic so I'm no guru, but I hope this helps answer your question.