It's a nice usability enhancement. When you've got a very likely-to-be-clicked link to a page with a large graphic or graphics, you can pre-load the graphic while your visitor is reading the earlier page. Then it's already in the cache and loads instantly if they click through.
At the very bottom of the body section, so your script doesn't interfere with the page that's being rendered:
Note: 'www' and 'hhh' are the width and height that you want to display these images.