Does your Desktop Wallpaper look similar to this?
If so, that means that the image you have set as your desktop wallpaper is too small for the resolution of your screen.
To prevent your wallpaper from becoming tiled, please choose a large image that is the same resolution as your computer screen.
You can find the resolution of your monitor by going to the Start button 
There you will see your screen resolution and what size your image needs to be.
In the example below, you will see the recommended resolution is 1280x1024:
Select an image that is large enough to fill the screen. Once you have downloaded an image you would like to set as your Wallpaper, right click the picture and select "Set as Desktop Background".