1.
XmlDocument doc = ...
XmlNode root = doc.DocumentElement;
XmlNode node = root.SelectSingleNode(yourXpathString);
node.Value = ...
2.try:
<xsl:template match="Pictures" >
  <xsl:for-each select="Picture"  >
  图片:<img src="{Picture}"/>
  </xsl:for-each>
</xsl:template>
3.
<Picture><Picture>007.jpg</Picture></Picture>
-->>
<Picture>007.jpg</Picture>