Yes it is
You can also use other file types such as png or jpg and it will work as well (example: ships.png). A special case is that you can replace ship graphics individually with files named ship1, ship2, etc... (add any extension)
If you want a list... well just check your COntinuum\graphics folder; most of them are pretty self-explanatory, same for the sounds. Just replace the .bm2 extensions with .bmp, and .wa2 with .wav, and open them to see what they are.
There are a few special cases, like colors.bmp and gradient.bmp, but a quick use of the search feature in the forum, or through the ASSS wiki, should give you the answer.