Image1 = new Image();
Image1.src = 'http://www.runinthesun.com/images/bookyour.gif';
Image2 = new Image();
Image2.src = 'http://www.runinthesun.com/images/bookyour_over.gif';

Image3 = new Image();
Image3.src = 'images/general/callback.gif';
Image4 = new Image();
Image4.src = 'images/general/callback_over.gif';

function changeimage(imagenum,newfilename)
{
   document.images[imagenum].src=newfilename;
}