Excellent question . . . easy answer:
1.) make a 256x128 jpeg using any image
2.) copy and paste image into FSX aircraft folder
3.) rename it thumbnail.jpg
4.) copy&paste the texture.cfg file from another repaint
5.) load FSX and view your masterpiece
There are prizes for anyone who can spot my Under Construction thumbnail in above imageP.S. Also works with FS9 aircraft added to FSX