I thank you for your very complete comment and the five stars :)

Thank you :) !

Actually the background is composed by a very wide gif animation. This animation was made using the level editor of the game Broforce. I created my own level with that special atmosphere which is not present in the original game. I recorded (with a lot of time and camera problems :)) the animation on my level and made some adjustments on GIMP. To enlarge the animation, i would have to create another piece of the scene which means to change my level and then record an animation of that piece and finally put together the old gif file and the new piece, which is practically, very hard and time consuming.

I made the background with the editor level of the Broforce (independant game) ;). Thank you for the five stars !

Merci pour ce commentaire très élogieux ! :)