MKN - I would (rightly or wrongly!) define a monochrome image as one that consists of various shades of a single colour. So you could fix the red and green channels, and then vary the blue from 0 to 255 (or whatever the pixel depth is: 8, 16, 32, etc.). For gray scale, the red, green and blue channels would be identical.
Using the above, a gray scale image would be considered a monochrome image, but a monochrome image would not necessarily be a gray scale image.
A binary image has two colours, typically black (0) and white (1). Though if 8 (or more) bits are used for the two colours and/or three channels, then you could have two other colours (of your choice) used to represent the binary image.
