The difference between Canvass ssize and Image size in Photoshop is that when you change the canvas size you don’t affect the contents of the layers while with Image size you force the layers contents to lock to the boundaries of the new document size.
Stock image credit:
Shot by: ~octoberjoy on deviantART
Available as free high resolution stock image at : http://octoberjoy.deviantart.com/art/Studio-portrait-1-266536152