Editing the FS2k2 road scenery textures?

After seeing the brilliant improvements to be got with the White Nights
download from Manfred Domandi, I thought I'd try a hand at editing some
of the files myself to liven up the road textures.
Big disappointment- I get FS2002 crashing when it goes from twilight to
night.
There seems to be some limitation what you can do with these files, but
I don't know what it is.

Has anyone been down that road and give me some clues?

Thanks
Tom