Use a rendering program like Bryce or Vue. they cost a little but not to much and you can use them for a variety of graphics. Terragen is great for photo realistic landscapes and is free for non commercial use.
All these have basic tutorials on how to get the images into 3dgs for skycubes and skydomes. Just run a search on the program you intend to use or post a req in the tut section.
Like many things with 3dgs it is a real simple process, but it takes a lot of practice to make it look good.
I also remember a tutorial that used Gimp to chop up a panoramic photo and create a sky cube texture out of that, but I think you would really have to enjoy photo manipulation to go that route.
