Here's a good approach from Lee Kelleher's blog, which is a "super safe" way to do it i.e. handles eventualities where the id isn't present etc:

<xsl:template match="/">
   <xsl:variable name="mediaId" select="number($currentPage/data[@alias='mediaId'])" />
    <xsl:if test="$mediaId &gt; 0">
     <xsl:variable name="mediaNode" select="umbraco.library:GetMedia($mediaId, 0)" />
        <xsl:if test="count($mediaNode/data) &gt; 0">
           <xsl:if test="string($mediaNode/data[@alias='umbracoFile']) != ''">
             <img src="{$mediaNode/data[@alias='umbracoFile']}" alt="[image]">
                   <xsl:if test="string($mediaNode/data[@alias='umbracoHeight']) != ''">
                       <xsl:attribute name="height">
                           <xsl:value-of select="$mediaNode/data[@alias='umbracoHeight']" />
                       </xsl:attribute>
                    </xsl:if>
                   <xsl:if test="string($mediaNode/data[@alias='umbracoWidth']) != ''">
                        <xsl:attribute name="width">
                            <xsl:value-of select="$mediaNode/data[@alias='umbracoWidth']" />
                        </xsl:attribute>
                    </xsl:if>
               </img>
          </xsl:if>
       </xsl:if>
   </xsl:if>
</xsl:template>

It looks a lot more intimidating than it really is, as it does check for loads of things rather than just 'get the media item from the id' but you should be able to easily trim it down if you do consider it overkill at all.

Hope this helps...

Thanks for the tip. Using

<xsl:variable name="mediaId" select="number($currentPage/ancestor-or-self::node [string(data[@alias='headerImage'])!=''] [1] /data[@alias='headerImage'])"/>

works like a charm :)

/ Niklas

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