Andrew Herd said it best in
THIS ARTICLE but if you put the new texture into the matching aircraft folder and place a new entry into the aircraft.cfg file the new texture will show up in the aircraft selction list.
[fltsim.xx] must be numbered in sequence
title=Boeing 747-400 (this must be different for each aircraft)
sim=Boeing747-400
model=
panel=
sound=
texture=ABC (texture.ABC folder)
kb_checklists=Boeing747-400_check
kb_reference=Boeing747-400_ref
atc_id=N747 your ident
atc_airline=Global Freightways
atc_flight_number=1123 anything will do
atc_heavy=1
atc_parking_types=CARGO (AF2 parking codes)
ui_manufacturer=Boeing
ui_type="747-400"
ui_variation="Global Freightways" (this shows in your FS9 list)
description="More than 30 years ago, the 747 made its first trip from New York to London.
Bill