[Fantastic Plastic Entertainment, Inc.] ” but I still get a file with the unchangeable default of 1 frame.”
You maybe doing something wrong, I use Bitmaps(.bmp) quite a bit, importing a still image should be as long as the basic default duration of 150 frames(unless the length has been changed in Preferences) and can be stretched as long as you want it, for instance I just changed the length of a 150 frame .bmp image to 4hrs by draging the right edge.