I recently created a placemark for the Sears Tower, Chicago, as part of a lesson on the characteristics of the central business district. I wanted to include a picture in the information window, as well a a link to the relevant wikipedia article.
The resulting placemark looks like this:
In order to add a picture to the information window, simply hunt around for a suitable image and then add it to the placemark information box, using a simple snippet of html.
Using the Sears Tower placemark for an example it works like this:
Replace all the code between the apostrophes with the URL of the image you want to include. This can be found by right clicking on the image and choosing Copy Image Location in Firefox, or Copy Shortcut in Internet Explorer.