I posted it to modding, because I need it for my mod.
Can you edit the scenery-gfx for online? (does it work online if you mod the sceneries, but the files have got the same lenght and width?)

No you can not. Or actually you can but I am not telling you how because you are not supposed to be able to edit them.

Yeah, there is a way to edit them withought the game realizng it, but some guy did that before, and we all considered it to be a "hack", so not a good idea. Just remake all the maps, but add your own sceneries (just load the .PMS files and clear all sceneries)

rename the sceneries and then edit them.