you have to have the pic posted on another website. Then you need to copy the URL for the actual pic. Usually its a very long string of characters that dont make sense. The easiest way is to right click the pick, then use "COPY IMAGE URL" - I use Safari. It will look something like. (I disabled the actual link so you can see what it would look like)
http:// ewtjaa.sn2.livefilestore.com/y1pWypUVYRJQGekf-8IIdIDZItGB-bCe8VBv-yOV3mwoDu2ZtqET-eB6Ndq10Q4f4Z8lWuzgTewOkXpQCITmPfM4kgC9BKxKbjq/DSC_0056.JPG?psid=1
I host my pic on Microsoft's SkyDrive but any site will do. Just make sure your browser supports "Copy Image URL"