Michael's pictures are available in several places, in addition to the GuideBooks and the Functions site.
If you're interested in something else, you can contact Michael.
Updated February 2005:
The newest GuideBooks postcard "Easter Eggs" will be first available at the Wolfram Research booth at the American Physical Society March Meeting, Los Angeles, 2005.
Updated December 2004:
There is now a series of GuideBooks postcards. They will be first available at the Wolfram Research booth at the Joint Math Meeting in Atlanta, January 2005.