This is a simple tutorial that creates a nice effect.
1. Open The image you wish give a black and white background.
Try to choose an image that has a relativly busy background or at least
something interesting in it.
2. Duplicate the layer then select the top layer.
3. Using the marquee (
) or Pen tool (
), crop out the area that you want to be black and white, and now delete
it by pressing delete.
4. You should now have 2 layers: The bottom layer with the
original image in color, and the top image which in my case is a woman.
5. In the layers panel select the bottom layer and go to image > adjust
> desaturate
) or Pen tool ( 
), crop out the area that you want to be black and white, and now delete
it by pressing delete.