Try this...
1. Right click on the empty background of your desktop.
2. In the popup list click "Personalize".
3. In this new window click "Desktop Background".
4. Now you should see all your images, etc., and on the lower left of the window there is a button under "Picture Position".
5. Click this button and you will get options for how your image will show. (fill, stretch, etc.).
6. Choose which one you want and then click the "Save Changes" button.