HTML Output navaigational graphics

Hi guys!
I want chunked my HTML output with xsltproc, DocBook v5.That works 
fine!But I want to have the navigation graphics instead of words like 
"Next" or "Previous". Any proposals, why it is not working?
I customize my stylesheet like this:

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

<!-- import Docbook XSL stylesheet -->
<xsl:import 
href="./docbook_config/docbook-xsl/docbook-xsl-ns-1.76.1/html/chunk.xsl" />
<xsl:import href="./docbook_config/config/documentation.xsl" />

	<!--
		 PNG images for HTML rendering
		 SVG is not well supported in several browsers
	-->
<xsl:param 
name="header.image.filename">images/png/logo_punktde.png</xsl:param>
<xsl:param name="admon.graphics" select="1" />
<xsl:param name="admon.graphics.path">images/png/</xsl:param>
<xsl:param name="admon.graphics.extension">.png</xsl:param>
<xsl:param name="callout.graphics.path">images/png/callouts/</xsl:param>

<xsl:param name="suppress.navigation" select="0"/>
<xsl:param name="navig.graphics.extension" select="'.png'"/>
<xsl:param name="navig.graphics" select="1"/>
<xsl:param name="navig.graphics.path">images/png/</xsl:param>
<xsl:param name="navig.showtitles">0</xsl:param>

</xsl:stylesheet>
-- 
Gruß Lwam Berhane

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