LGSL looks for images in the following order:

/lgsl_files/maps/{TYPE}/{GAME}/{MAPNAME}.jpg
/lgsl_files/maps/{TYPE}/{GAME}/{MAPNAME}.gif
/lgsl_files/maps/{TYPE}/{MAPNAME}.jpg
/lgsl_files/maps/{TYPE}/{MAPNAME}.gif

See the included readme for details on {MAPNAME} filtering,
and how to find out a servers {TYPE} and {GAME}.
