within my xsl I have the following basic rollover functionality

----
<a href="about.asp?page=1"
onmouseover="MM_swapImage('a1','','/images/about/about1_on.gif',1)"
onmouseout="MM_swapImgRestore()"><img src="/images/about/about1.gif"
border="0" name="a1"/></a>
----

I want to add a parameter to the image path, which is straightforward if its
just around the <img> tag:

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<xsl:attribute name="src"><xsl:value-of
select="$ImgDir"/>about/about1_on.gif</xsl:attribute></img>
----

but what within javascript statements? like in the mouseover state above?
how do I add the $ImgDir into the image path?

<a href="about.asp?page=1"
   onmouseover="MM_swapImage('a1','','{$ImgDir}about/about1_on.gif',1)"
   onmouseout="MM_swapImgRestore()">
  <img src="{$ImgDir}about/about1.gif"
       style="border:0" id="a1"/>
</a>