popup image link requires right-click in IE

I have a page with a:hover popups containing links -- some text, some
images.

http://www.cs.northwestern.edu/~riesbeck/popups/

The text links work fine. Roll over a building, click the popup that
appears, and off you go.

But in IE 7 (and 6 I'm told), clicking the popup image that appears
for the small brown building in the lower right does nothing. You have
to right-click to open a context menu and choose "open link." No such
problem in Firefox, Opera, or Safari for Windows.

Is this a CSS problem, or some policy IE has about images that are
links in popups?