Acrually, that's not what they are doing. There's this big image:
http://www.greyhawkeatthelake.com/images/Plat2.jpg . Then they have positioned images of the different colored dots over that image och do an ordinary onmouseover/onmouseout on each one of them.
HTML
<div id=Lot1div style="position:absolute; top:54px; left:401px;" onMouseOver="doOverlib(this, 1);" onMouseOut="nd();"><a id=Lot1href href="#"><img id=Lot1 src="../images/maps/status/status_availablehome.gif" border=0></a></div>