This is a client side javascript app which maps the image into a bunch of regions. Take a look at the source for the page with the map on it. About halfway down, you'll see 50 lines like these.
<map name="usa">
<area shape="rect" coords="215,4,331,29" href="#" onClick="document.myform.searchstate.value='CD';this.href='javascript:document.myform.submit()'" alt="CANADA">
<area shape="rect" coords="539,113,563,126" href="#" onClick="document.myform.searchstate.value='NJ';this.href='javascript:document.myform.submit()'" alt="NJ">
<area shape="rect" coords="541,87,564,99" href="#" onClick="document.myform.searchstate.value='RI';this.href='javascript:document.myform.submit()'" alt="RI">
Notice that each state has been defined by a set of rectangular coordinates and when the mouse is clicked within that rectangle, the searchstate value is set to whatever two character state is contained within the rectangle.
Then, at the bottom of the html page, you'll see that the store listing script is called with the value in searchstate.