When you want to display images dynamically, if you want to make sure they actually exist, you can do fairly easily directly in XSL:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:php="http://php.net/xsl" version="1.0">
<xsl:output method="xml" encoding="UTF-8" indent="yes" />
<xsl:template match="root">
<html>
<head></head>
<body>
<!-- We suppose node 'id' contains part of the filename -->
<xsl:variable name="imgSrc" select="boolean(document(string(concat('http://www.sillysmart.org/img/users/', id, '.jpg'))))" />
<xsl:choose>
<xsl:when test="$imgSrc">
<img src="{concat('http://www.sillysmart.org/img/users/', id, '.jpg')}" alt="{id}" />
</xsl:when>
<xsl:otherwise>
<img src="http://www.sillysmart.org/img/404.jpg" alt="Not found" />
</xsl:otherwise>
</xsl:choose>
</body>
</html>
</xsl:template>
</xsl:stylesheet>
