Swap Images

I'm trying to change the images between "active", "hover" and "pressed," but it isn't working.

Here is my code:

<script language="JavaScript" type="text/javascript">
<!--
function mouseover(source)
{
// alert("source over is '" + source + "'");
document[source].src = eval("../images/" + source + "f2.gif");
}
function mousedown(source)
{
// alert("source down is '" + source + "'");
document[source].src = eval("../images/" + source + "f3.gif");
}
function mouseout(source)
{
// alert("source out is '" + source + "'");
document[source].src = eval("../images/" + source + ".gif");
}
-->
</script>

and it is called from within HTML by this:

<a href="belts2.php?belt=yellow">
<img src="../images/Yellow.gif" name="yellow"
onMouseOver="mouseover('yellow')"
onMouseDown="mousedown('yellow')"
onMouseOut="mouseout('yellow')"
alt="Yellow Button" id="yellow" border="0"></a></td>

I haven't done this in years and years so I need a quick refresher on what I'm doing wrong.