Try,.
<img width="350" height="350">
<xsl:attribute name="src"><xsl:value-of select="//*/Source"/></xsl:attribute>
</img>
or
<img width="350" height="350">
<xsl:attribute name="src"><xsl:value-of select="MapImage/Source"/></xsl:attribute>
</img>
or
<img width="350" height="350">
<xsl:attribute name="src"><xsl:value-of select="./Source"/></xsl:attribute>
</img>
its all depends on how you call the template and from what node.
Check this xpath/xsl tutorial from
http://www.w3cshools.com , pretty good.