Device-independent bitmaps (DIBs) are used to deal with more than one bit-per-pixel bitmaps. The implementation in the PostScript driver uses the PostScript image operator on black and white devices and the colorimage operator on color devices to render multiple bit-per-pixel images. (Notice, however, that this makes the output printer dependent.)
Applications that want to print multiple bit-per-pixel images should use the SetDIBitsToDevice or StretchDIBitsToDevice function to render these images. GDI will simulate these if the driver does not support them.