Event.observe(window, 'load', function(e) {
    $$('a img').each(function(img) {
        var a = img.up();
        if (img.readAttribute('hover-src')!=null)
	        a.observe('mouseover', hover_mouseover).observe('mouseout', hover_mouseout);
    });
});
function hover_mouseover(e) {
    var img = this.down('img');
    var src = img.src;
    img.src = img.readAttribute('hover-src');
    e.stop();
}
function hover_mouseout(e)
{
    var img = this.down('img');
    img.src = img.readAttribute('normal-src');
    e.stop();
}