xsl:variable question

Hello,

I have a construction which looks as following:

<xsl:variable name="tree-structure-new-branch">
    <xsl:choose>
        <xsl:when test="not(position()=last())">
            <img width="16" height="16"
                 alt="Der Zweig geht nachher noch weiter"/>
        </xsl:when>
        <xsl:otherwise>
            <img width="16" height="16" alt="Der Zweig endet hier"/>
        </xsl:otherwise>
   </xsl:choose>
</xsl:variable>

I have traced through this piece of code, it seems to follow my xml tree
well. But the variable I define here is empty after leaving this xsl
structure. Why? What should I change to store that img element in the
variable?

Regards,
Kai