if (window.addEventListener)
{ 
   window.addEventListener("load", btg, false);
}
else if (window.attachEvent)
{ 
   window.attachEvent("onload", btg);
}

var picarray;

function btg()
{
   //load hover images
   picarray = new Array(4);
   
   picarray[0] = new Image(150,150);
   picarray[0].src = 'images/susp_hover.png';
   
   picarray[1] = new Image(150,150);
   picarray[1].src = 'images/laser_hover.png';
   
   picarray[2] = new Image(150,150);
   picarray[2].src = 'images/past_hover.png';
   
   picarray[3] = new Image(150,150);
   picarray[3].src = 'images/cat5_hover.png';
   
}

function btg_hover(pic,index)
{
   var tmp = pic.src;
   
   pic.src = picarray[index].src
   
   picarray[index].src = tmp;
}