This was the code I ended up using inside one of the XSL samples.
<xsl:when test="FieldName[starts-with(., 'Coll')]">
<xsl:variable name="imageName" select="FieldValue"/>
<img src="T:\SEFIS2010_GVdata\SEFIS Video\{$imageName}.jpg" width="192" height="108"/>
</xsl:when>
Edit
This is the full XSL code where this is used
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:fo="http://www.w3.org/1999/XSL/Format">
<xsl:template match="/">
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8"/>
</head>
<body>
<xsl:variable name="nameCol" select="FieldsDoc/Fields/Field/FieldName"/>
<table border="1" width="300" cellpadding="5" cellspacing="0">
<tr bgcolor="#9cbce2">
<xsl:if test="string-length($nameCol) != 0">
<th width="50%" align="left">Field Name</th>
</xsl:if>
<th width="50%" align="left">Field Value</th>
</tr>
<xsl:variable name="index" select="1"/>
<xsl:for-each select="FieldsDoc/Fields/Field">
<tr>
<xsl:if test="string-length($nameCol) != 0">
<td bgcolor="#D4e4f3">
<xsl:value-of select="FieldName"/>
</td>
</xsl:if>
<td>
<xsl:choose>
<xsl:when test="FieldValue[starts-with(., 'www.')]">
<a target="_blank"><xsl:attribute name="href">http://<xsl:value-of select="FieldValue"/>
</xsl:attribute><xsl:value-of select="FieldValue"/>
</a>
</xsl:when>
<xsl:when test="FieldValue[starts-with(., 'http:')]">
<a target="_blank"><xsl:attribute name="href"><xsl:value-of select="FieldValue"/>
</xsl:attribute><xsl:value-of select="FieldValue"/>
</a>
</xsl:when>
<xsl:when test="FieldValue[starts-with(., 'https:')]">
<a target="_blank"><xsl:attribute name="href"><xsl:value-of select="FieldValue"/>
</xsl:attribute><xsl:value-of select="FieldValue"/>
</a>
</xsl:when>
<xsl:when test="FieldValue[starts-with(., '\\')]">
<a target="_blank"><xsl:attribute name="href"><xsl:value-of select="FieldValue"/>
</xsl:attribute><xsl:value-of select="FieldValue"/>
</a>
</xsl:when>
<xsl:when test="FieldName[starts-with(., 'Coll')]">
<xsl:variable name="imageName" select="FieldValue"/>
<img src="T:\SEFIS2010_GVdata\SEFIS Video\{$imageName}.jpg" width="192" height="108"/>
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="FieldValue"/>
</xsl:otherwise>
</xsl:choose>
</td>
</tr>
</xsl:for-each>
</table>
<br/>
</body>
</html>
</xsl:template>
</xsl:stylesheet>