There are many times when a black and white image may be more suitable than a colour one.

• You may be printing this image on a black and white printer
• The user may find a black and white image easier to 'read' than a coloured one ?
• You have a machine that works in 256 colours rather than 16-bit or 32-bit colour. (You can check the colour from the settings tab of the desktop properties menu )

If you want to do this you will need to set the image to 'greyscale' (not black and white, which does not include greys). Some image software has an option under the image-editing menu for this, other software will handle it in the save dialog box.