Hi again,
I reply to myself, I got it working using the following (simplified) XSL :
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:csw="http://www.opengis.net/cat/csw/2.0.2"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:gmd="http://www.isotc211.org/2005/gmd"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gts="http://www.isotc211.org/2005/gts"
xmlns:gco="http://www.isotc211.org/2005/gco"
xmlns:geonet="http://www.fao.org/geonetwork">
<xsl:output method="html" doctype-system="about:legacy-compat"/>
<xsl:template match="/root">
<xsl:apply-templates select="gmd:MD_Metadata"/>
</xsl:template>
<xsl:template match="gmd:MD_Metadata">
<html>
<head>
<title>Metadata</title>
</head>
<body>
<img src="{root/resourceUrl}/map_header.png" alt="map header" width ="700"/>
<p>title</p>
<h1><xsl:value-of select="gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:title/gco:CharacterString"/></h1>
</body>
</html>
</xsl:template>
</xsl:stylesheet>
Hope it can help someone with the same problem.
However, I still can't get my image to display. Any idea ?
(By the way, if I'm not mistaken, there's a typo in the doc here http://www.geonetwork-opensource.org/manuals/2.8.0/eng/users/managing_metadata/formatter/index.html#formatter : the tag name is resourceUrl and not resourceURL).
Thank you !
Julie.