GPScorrelate writes EXIF GPS tags to JPEG photos using a GPX tracklog.
Both a command line utility (gpscorrelate) and a GTK+ application 
(gpscorrelate-gui) are provided. 

The date/time of each photo is correlated with a GPS tracklog and the 
photos are tagged (or "geocoded") with appropriate EXIF GPS tags.  For
more information please see the GPScorrelate homepage.

The tracklog must be in GPX format.  One suitable utility for downloading 
tracklogs and translating them to GPX format is gpsbabel, which is also
available at SlackBuilds.org.