Step 1:- First of all open photoshop cs5 and open an image of which background you want to change.

Step 2:- After that Select the Quick Selection tool from the toolbar in leftbar. You can also choose that tool simply by pressing W.

Step 4 :- After selecting that area click on the refine edge and set the parameters according to your image.

Step 5:- Now Open that image in which we want to add this image and we want to use that as a background.

Step 6:- At the end we will drag that selected part of the first image into it and adjust the position of that image.If there is any problem of size then press the shortcut key Ctrl+T and adjust the size of that selected image and place at that position where you want to place it.