Creating a skybox from 6 diff. pictures

I’ve a lot of different skyboxes coming each as 6 separated pictures (one for each side of a cube).
So I’m looking for the simpliest way of creating a skybox in Blender … using one of these 6 images group.
Do I need to built a single picture containing all the six ones, then to UV map that picture to a cube, and then set that picture/texture as an envmap in Blender ?
If yes do you know a free tool that could make one big picture of 6 little ones ?
Any ideas ?