All graphic files must be in a format that can be loaded by the python imaging library and they must have a valid palette. You can chose either the default (DOS) palette (recommended as it contains a few more colours) or the legacy (windows) palette.
Palette files for several editors:
- Default (DOS) palette for GIMP
- Legacy (Windows) palette for GIMP
- Default (DOS) palette for Photoshop
- Legacy (Windows) palette for Photoshop
- (another collection of palette files on the #openttdcoop DevZone)
Note: Windows versions of NML do NOT support graphics in the pcx format due to missing support within the PIL.