<SCRIPT>

if (document.images) {
     arrowRed = new Image();
     arrowRed.src = 'images/redArrow.gif';

     arrowBlue = new Image();
     arrowBlue.src = 'images/blueArrow.gif';
}
else {
     arrowRed = "";
     arrowBlue = "";
     document.arrow = "";
}

</SCRIPT>

<BODY>

<A HREF="../index.htm"
  onMouseOver="document.arrow.src=arrowRed.src"
  onMouseOut="document.arrow.src=arrowBlue.src">
<IMG SRC="images/blueArrow.gif" NAME="arrow">
</A>

The if part of the code for example, creates the arrowRed image object, then using the .src property fills it with the redArrow.gif.

The else part of the code tells the "old" browsers what to do, in order to avoid the error messages. So dummy variables are created & set to "". These variables won't do anything except be "empty" placeholders.