What version of Gamestudio do you have? 6.31? 6.20? And it's 16.8 million (16,777,216, also referred to as a 24-bit BMP or truecolor image).

Can't you open the BMP file and save it as a TGA? You don't need to make any changes or add an alpha channel. If you don't have a program that saves as TGA, get GIMP. I use it constantly. I used it to make the animation in my avatar (updated just today).