master.xml

<data>
  <images>
    <entry>
      <image>myimage1.jpg</image>
    </entry>
    <entry>
      <image>myimage2.jpg</image>
    </entry>
    <entry>
      <image>myimage3.jpg</image>
    </entry>    
  </images>
</data>

master.xsl

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

  <xsl:output method="xml" indent="yes" />
  <xsl:variable name="quote">
   <xsl:text>"</xsl:text>
  </xsl:variable>
  <xsl:template match="/">

   <xsl:variable name="imagelist">
     [<xsl:apply-templates select="/data/images/entry/image" mode="imagelist"/>]
   </xsl:variable>
    <script>window.imageslist = <xsl:value-of select="$imagelist"/>;</script>
  </xsl:template>
  
  <xsl:template match="entry/image" mode="imagelist">
    <xsl:value-of select="concat($quote, .,$quote)"/>
    <xsl:if test="position() != last()">,</xsl:if>
  </xsl:template>
</xsl:stylesheet>