I know what you mean, as i did the same thing. im not sure why they cant be viewed, but there are some good blank textures for all the stock aircraft @
http://www.simviation.com/fs_paintkits1.htmand the pages that follow. These do show up on ms paint. in addition, most downloaded aircraft have viewable texture files.
however, fs2002 textures largely cannot be edited just using ms paint. unfortunately, youll need to buy paint shop pro or a similar graphics program to repaint

. im not quite sure why, but ms paint never relays the textures onto the model, (no textures are displayed)
daz