Hi, right click on an empty space of your desktop. Click the last point ("modify" or similar, I translate what I see on a German windows, what you see can be different). Scroll to the bottom of the opened window. Click "desktop background picture" (or similar). That opens the "desktop background" window, scroll again to the bottom and pick a position for your picture. "Centred" is clearly not what you want, test the other options: The picture can be stretched horizontally (automatically adjusted height) or vertically (automatically adjusted width). Normally you should not modify height and width simultaneously, because that destroys the aspect ratio of your picture (people on the picture would look unnaturally fat or slim).
If that doesn't help please post a complete description of your problem.
