<?xml version="1.0" encoding="UTF-8"?>
<Document>
<Placemark>
<TimeStamp><when>2009-09-07T09:17:06Z</when></TimeStamp>
<Point><coordinates>10.688447,60.795394</coordinates></Point>
<ExtendetData>
<Data name="footprintid"><value>1a661a0787b4d067ce30aa870114e3a0</value></Data>
<Data name="image"><value>somefilename</value></Data>
</ExtendetData>
</Placemark>
<Placemark>
<TimeStamp><when>2009-09-07T09:17:06Z</when></TimeStamp>
<Point><coordinates>10.688447,60.795394</coordinates></Point>
<ExtendetData>
<Data name="footprintid"><value>1a661a0787b4d067ce30aa870114e3a0</value></Data>
<Data name="image"><value>somefilename</value></Data>
</ExtendetData>
</Placemark>
</Document>

xpath -e '/kml/Document/Placemark/ExtendedData/Data[@name="image"]/value/text()'