NOTE: The source of the problem has been determined. Apparently, Photoshop CS2 (which I am using) handles .png files differently than previous versions of Photoshop (for which the Modmaker's Guide instructions were written).
For those who might have occasion to use Photoshop CS2, or anyone who just happens to be interested, a working method for producing ship silhouettes in CS2 can be found at the
ISF: Mod Surgery forum, in the thread "Ship Silhouettes: Transparency?"