Argh, I was hoping you wouldn't say "Gimp" since this software is really complicated. 
But it worked. In fact, when I opened the picutre on Gimp, the alpha channel was already set ! I saved the picture again, tried to lanch the game, and the switches worked well and had a transparent background. I suppose I can do the same operation with bushes.
Thanks for your explanations and your patience with my little problems.

But it worked. In fact, when I opened the picutre on Gimp, the alpha channel was already set ! I saved the picture again, tried to lanch the game, and the switches worked well and had a transparent background. I suppose I can do the same operation with bushes.
Thanks for your explanations and your patience with my little problems.
:





) but it's a little strange.

) : in which section can I ask some help about a Lua script (related to a map) which doesn't work ? I thought it was the "Development" section, but I'm afraid that it's for tutorials only.