There used to be a harder way, involving square brackets. Now you just point to the URL of the pic. Edit this page to see how.
You must host the pic elsewhere - only Ward can save a pic on his server.
Egad! How does it know that it is an image and not a link?? Does it actually do a head request to check the content-type or just grep for (gif|jpg)? Seems a bit suspect to me... but heck, if it works 95% of the time maybe it just doesn't matter all that much ;-).
A look at the code in http://c2.com/cgi/hp?WikiInHyperPerl and find-in-page for gif has two hits, the second indicates the RegularExpression only for .gif at this time. Thanks be to Ward.