Testing beats guessing 
The images on the left is 24 bit bmp (RGB)
The middle is a 32 bit bmp.
And the right is a 16 bit CMYK (jpeg, as bmp doesn’t support cmyk)
(For the record: it doesn’t like 8 bit cmyk either)
Rhino doesn’t like cmyk or above 24 bit. It doesn’t throw an error, it just shows the image as white or black.