How to convert a B&W image to Grayscale using C#?

Question

I want to use the ImageStatistics class of the AForgeImaging namespace:

  ImageStatistics im = new ImageStatistics(img);

The problem I am having is that the parameter (img above) must 8 bpp grayscale or 24/32 bpp color.

What if I start with a B&W image ? How can I use this class?

Ultimately, my question is: How can I (efficiently) convert a B&W image to Grayscale (with or without the AForge library) ?