Calling a Java extension from XSLT using Saxon B

I am not a Java programmer, but I would like to use an extension to check for
the existence of an image file referenced by an XML file (as I believe that is
the only way to check for the existence of a non-XML file). This topic has
been discussed a lot on various aliases, and despite my best efforts, I'm
unable to unravel the mystery. 

I've included what I believe to be relevant information.

Error Message:
--------------
Error at xsl:value-of on line 15 column 48 of nxs-test-image-exists.xsl:
  XPST0017: XPath syntax error at char 20 on line 15 in
{nxs:fileExist(@osrc)}:
    Cannot find a matching 1-argument function named
{java:FileExist}fileExist()
Failed to compile stylesheet. 1 error detected.

Saxon-B 9.1.7 J stored in xmlsrc/xTools/saxon9.
-----------------------------------------------

Java program (compiled and the FileExist.class file stored in
xmlsrc/xTools/saxon9/classes)

/* Source: http://p2p.wrox.com/xslt/71563-xslt-check-if-directory-exist.html
*/

import java.io.File;

public class FileExist {

    private static boolean exist;

    public static boolean fileExist(String myfile) {

        exist = new File(myfile).exists();

        return exist;

    }
}

XML Snippet for img element
---------------------------

...
<img osrc="../../images/1982.jpg" />

...

XSLT
------

<?xml version="1.0" encoding="UTF-8"?>

<xsl:stylesheet version="2.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:nxs="java:FileExist">

<xsl:output method="xml"/>

<xsl:template match="/">
                <xsl:apply-templates />                                    
</xsl:template>

<xsl:template match="img">
                <xsl:value-of select="nxs:fileExist(@osrc)" />
</xsl:template>
    
</xsl:stylesheet>

Korn Shell script/command line call:
------------------------------------

# define saxon command line
saxon="java -classpath xTools/saxon9/classes; -jar -Xmx1G
xTools/saxon9/saxon9.jar -tree:tiny -dtd:off -ext:on "
# call transform
$saxon deliverables/help/helpFile.xml  xslt/nxs-test-image-exists.xsl >
imageerror.xml


Thank you for any insights you might offer.

Blaine