Right-click any open part of the desktop screen.
Click Personalize.
Click Desktop Background.
Click Browse.
Find the location of the picture you want to use and click OK.
Put a check mark in the picture you want to set as the Windows background.
Choose from Center, Tile, Fit, Fill or Stretch for the Picture position option, to select how the picture is displayed as the background on the desktop.
Note: If you have Windows 7 Home Basic or Windows 8 Basic on your computer, the "Personalize" option may be greyed out or not available at all. To change the desktop background, click the Start menu button and type change desktop background in the Search box. In the returned search results, click the "Change desktop background" option. You can then use the steps above, starting with step 3, to change the Windows background.
How do I make a small image tiled across the background?

Within the desktop or background properties window, a user will be able to customize the layout of their image by selecting Center, Tile, or Stretch. Below is a brief explanation of each of these settings.
Center - Centers the image in the middle of the desktop. Doing this enables the user to only have one image set as their background instead of having the image tiled. If the image is smaller than your computer resolution, then you may have a solid color border around the image.
Tile - Tiling an image takes a smaller image and duplicates it multiple times across the Desktop.
Stretch - Stretching an image sets a single image across the whole screen. The smaller the image size, the more distortion you will notice when stretching an image. It may also result in pixelation of that image.