Here is a short and sweet tutorial on creating an awesome fall / autumn landscape, complete with tree and falling leaves. This tutorial uses a mix of GIMP and Inkscape techniques, but should be easy to follow for just about everyone.
Here is a neat howto from Libre Graphics World on how to use Inkscape and the GIMP to create a simple, colourful acrylic painting effect from a boring, lifeless photo.
Yet another awesome step-by-step from kalaalog demonstrating how to illustrate a palm tree. The result is quite impressive.