<root>
	<users>
		...........
		<user>
			<id>masha</id>
			<pic> 12432 .jpg</pic>
		</user>
		...........
		<user>
			<id>serj</id>
			<pic> 109868 .jpg</pic>
		</user>
		...........
	<users>
	..................
	<comments>
		<comment user="masha" date="20.12.2010 15:35">
			<text>Ля-ля-ля</text>
			<comment user="serj" date="20.12.2010 16:10">
				<text>Ты права!</text>
			</comment>
		</comment>
	</comments>
</root>

<xsl:template match="comment">
	..................................
	<img src="files/{/*/users/user[id='значение']/pic}" />
	..................................
</xsl:template>