First, you need to upload your picture(s) to the web (beware some free hosts like geocities do not allow hotlinked photos). You can't post it directly from your PC. Most internet accounts these days come with a free web site account. Once the picture is uploaded to the web, point to the picture's location in the "Image URL" box below.

examples:
http://www.yoursite.com/gallstones.jpg
http://www.yoursite.com/pictures/gallstones.jpg