Add an attribute to a selected node from the source XML

I am trying to add an attribute to a node selected from the source XML. My initial attempt is below, but I think I am misund开发者_如何学编程erstanding the concept. Do I need to load the node into a variable first, or is there another way to do this?

Source XML

<root>
   <BigImage>
      <img alt="Flower" src="/flower.jpg" />
   </BigImage>
</root>

Desired output

<img alt="Flower" src="/flower.jpg" class="image-left" />

Current (incorrect) XSLT

<xsl:template match="root">
  <xsl:copy-of select="./BigImage/node()" />
    <xsl:attribute name="class">image-left</xsl:attribute>
</xsl:template>

Current (incorrect) output

<img alt="Flower" src="/flower.jpg" />

This transformation:

<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output omit-xml-declaration="yes"/>
 <xsl:strip-space elements="*"/>

 <xsl:template match="img">
   <img class="image-left">
    <xsl:copy-of select="@*"/>
   </img>
 </xsl:template>
</xsl:stylesheet>

when applied on the provided XML document:

<root>
    <BigImage>
        <img alt="Flower" src="/flower.jpg" />
    </BigImage>
</root>

produces exactly the wanted, correct result:

<img class="image-left" alt="Flower" src="/flower.jpg"/>