embedding xsl within javascript

hi there, my first post here - how exciting...   ;)

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:

----
<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?

many thanks

rob