GIMP DDS plug-in needed to save to cubemap DDS format:
This script will automatically create, crop, reposition and rename all layers needed to save a cubemap with the GIMP DDS plug-in from a regular skybox image of choice.
How to use
Create a new file in the GIMP (just about any parameters will do). You'll find the script under Filters > Generic. Everything else is pretty straight forward from there.