Arwen-Undomiel.com's tutorial page:  This isn't GIMP specific, but it is often possible to adapt her tutorials for GIMP.

 

Gallery at Arwen-Undomiel.com:  The source for all of the LotR screencaps

 

Thread on A-U.com's forum about using GIMP:  Includes a few tutorials.

 

 

 

 

 

 

 

 

 

As a bit of shameless self-promotion, my Tolkien site.  It does actually have some graphics which relate a bit to what I talk about. /miserable excuse