If the photos are on your computer, just click on the "Additional Options" at the bottom of the box, and click on the Browse button there.
If it is on the 'net, go to the picture, and copy the URL from the top of the page there (that's the "www" address- like this-
http://www.atvflorida.com/forum/index.php?action=post;topic=5264.0;num_replies=0and then click on the BBC tag of the earth on it once, then paste the URL in between the
http:// things. It's easy once you do it a couple of times.