What you do is just get the link for your pic.... go to the pic on the web and right click it. Then go to the bottom of the list where it says "properties" and it will give you the URL for the pic.
Just place that URL - http:/www.whatever.com/nudie.jpg - for example.... between these things:
If your pic is not posted on the web somewhere already then I think you would have to post it to a site and then link to it... I could be wrong about that though. Someone will correct me if I am wrong. I have, so far, only ever posted to things already up on the net.