ah! In my tests, the problem appeared to the left and right, and I was thinking to images Left and Right... In fact, it was just the original orientation (direction) of the Player, because the problem was just back and front...
Thanks for help Green Gandalf
For info, I used night.x file (in the "night" skybank) to try this new skybox. The solution to generate an original file.x for these textures is still unknown. I do not despair !
Cheers