(The below excerpted from nocturne@mit.edu's how to imagemap page: )
default URL 
rectangle (x1,y1) (x2,y2) URL 
circle (x,y) r URL 
For more info on CERN's imagemaps, try:
http://www.w3.org/hypertext/WWW/Daemon/User/CGI/HTImageDoc.html
-rei