img0_on = new Image();
img0_on.src="menu1on.jpg";
img0_off = new Image();
img0_off.src="menu1off.jpg";

function over_image(parm_name)
    {
        document[parm_name].src = eval(parm_name + "_on.src");
    }
function off_image(parm_name)
    {
        document[parm_name].src = eval(parm_name + "_off.src");
    }

<a href="http://www.blabla.com/menu1" onmouseover="over_image('img0');" onmouseout="off_image('img0')"> <img src="menu1off.jpg" border="0" name="img0"> </a> <br>