Without question, the biggest help I have in designing grapics
elements is Kai's Power Tips
and Tricks online. Although the tips are designed primarily for
Adobe Photoshop, the techniques and theory adapt exceedingly well to
the GIMP. There are also several great tips on the MetaTools homepage, as well as
some interesting biography on this image manipulation master.
The GIMP home page has a set of tutorials for
GIMP-specific actions. Ignore the cross-fading tutorial; you can do that in
one step with the Composite chanop. (sorry Toby)