The following instructions cover making a photo that you have found on the Internet your desktop background. Instructions differ by browser. However, the first step is always the same.
The first step is to click on the thumbnail so that the full-sized image appears.
The second step depends on your browser.
Right-click the full-sized image and then select “Set as Background”.
Right-click the full-sized image and then select “Set As Desktop Background…”.
Right-click the full-size image and then select “Save Image As…”. Save the image to your desktop, and then from the Windows Desktop, right-click the image and select “Set as Desktop Background”.