I recently made a custom scenery. I made it quite small, and set the background the same light green as the others. Then saved it as a .bmp into the "Scenery-Gfx" folder.

I opened the map editor and scattered the scenery in different locations with different sizes, transparencies, and colors. I saved the map into the appropriate folder. I then gave the scenery and map to my friend to test, whereas he put the files into the correct folders as well. Then I created the game, I could see everything fine, but when he joined he got this error message, more or less:

"Original scenery cannot be modified"

And he saw nothing but the background.