Slice tool and use of dreamweaver

Today, I had a demo no how to use the slice tool in photoshop, this allowed me to select whatever I wanted on The image. When the web page was opened from how it was originally set. It was positioned at the top left of the screen, small and with a white background. In dreamweaver I was able to change the background colour to black and also to move it into the centre, so now when the web page was opened, it was in the centre of the web page with a matching black background. These are examples how it looked before and after: