There are a few ways to go about changing your windows desktop background image and conveniently, they are both very easy!
The first way, is to locate the image you would like to use for your desktop background. Make sure that the file is big enough; otherwise windows will stretch it to fit your screen resolution which can result in the image not looking as nice as you would like.
Next, double click the image to open it up in Windows Picture and Fax Viewer.
Once you have the image visible, right click on it and then click 'Set As Desktop Background'. If the image is really small windows will tile it. If the image is larger, it will either scale it to your screen or center it.
The other way you can easily set your desktop wallpaper is by first right clicking on your desktop, and then clicking properties.
You will be brought to your computers 'Display Settings'. At the top of the display settings you will see multiple tabs, click the Desktop one. On the right you will see a 'browse' button.
Click the browse button and locate the image you would like to use as your desktop background. After you've selected the image to use, beneath the browse button you will see a position drop down box. You will be given three options, center, tile and stretch. Play around with this option to find the option that looks best with your new desktop wallpaper.