The mask method comes in handy when you have reflections or portions of your object which are white or close to white. If so, it is difficult to cut the white bits out with your paint program. Using the mask is simple. Take the background image you want to use. Now place your object image as a layer over it and place the mask over the object image as a new layer. Then on the mask layer simply select all the black areas. Then, drop down to your object image layer (with the black areas still selected) and delete. You can then delete the mask layer if you wish. Your end result should be your object sitting over your background.

Could be worse, could be raining.