To change the desktop wallpaper images on a Windows 7 computer, go to Start, open the Control Panel, type "desktop background" in the search field, and click Change Desktop Background. Select a new wallpaper from the list of images that appear. If you want to set a custom image, click Browse, navigate to the folder with the custom image, and double-click it to set it as the desktop wallpaper.
If you wish to specify the positioning of the image on the screen, go to Picture Position, click on the arrow to choose between the available layout options, and click Save Changes. A shorter way of setting a custom image as the desktop wallpaper is simply navigating to it, right-clicking it, and clicking Set As Desktop Background.
To change the wallpaper on a Windows 8.1 computer, swipe in from the right edge of the screen, click Search, type "desktop background" in the search field, and click Change Desktop Background. Select the image you want to set as the wallpaper. If you want the desktop background to show two or more images in slide show format, tick the check boxes for the images you want to feature in the slide show, and click Save Changes.