Adobe Photoshop CS5.1 - Contract and Expand functions

Hello everyone,

I am trying to alter few photos in Photoshop CS5.1 and, for the most part, I managed to do it
However I hit a wall trying to use Expand or Contract functions under Select-Modify-Expand or Select-Modify-Contract.  
What I am trying to accomplish?  Contract the picture in such a way that the subject will look like he was sitting further from the camera.  To my knowledge (very basic in Photoshop) the Contract and Expand are the best functions to accomplish this because both keep the proportion of the picture unchanged so the picture, although smaller, looks very natural.

So I read few article, watched a Youtube short tutorial and follow the instructions .  For testing purposes I use a picture 2" x 3.2"   (50x80 mm)  , kind of ID picture.    Here are the steps I followed

1.  Highlight the background of the picture using either Magic Wand or Quick Selection Tool  (not sure what is the difference, I tried them both)   . Got the dotted line along the contour of the head and shoulders of the person

2.   On menu bar  click on Select - Inverse . By doing this I left the background out so implicitly I selected the person  !  

3. Click on Select - Modify - Contract

4. In Contract by, I entered the numer of the pixels  , for example 15

5. The dotted line around the head and shoulders moved inwards. (great, I expected that)

6.  Now what ? What is the next step , where do I have to press to see the real result of the steps above ? Tried many things with no luck, the dotted line is still there in its new position and nothing happen, the picture did not shrink.  Is like is waiting for me to finish , but I don't know how

Thank you all in advance for your help !

Cheers !