Creating a skybox using a panoramic image

[Guessmyname]

Anyone know how?

I'm trying to create a skybox using the 'fullskies' section of CGTextures, but I've no idea where to even start (all the tutorials I've found assume the sky image is already broken up in the six pictures). I don't know how to cut up the image to get the top face out of the picture. Any advice, tutorials, methods, programs (preferably free ones...) would be greatly appreciated (seriously, been spending hours looking here...)