I can't see anything obvious. Check that the mac you are testing on doesn't have Javascript disabled (e.g. do an alert("hello world") ). If that is not the problem, it may be that mac IE 5.1 doesn't support .png, so see if the symptoms change using gif. You could try other ways to write your assignments, such as document.Home.src='HomeB.png' (although I don't see anything wrong with the way you have it).
Note that you could do the mouse-over effect using css instead.