To install a free holiday wallpaper on a computer running Windows 7, download the wallpaper, go to the Start Menu, click Control Panel, search for "desktop background," and click on Change Desktop Background. Click Browse, and double-click on the holiday wallpaper you downloaded to set it as your desktop background.
If you are using a Mac, download the holiday wallpaper, store it in a wallpaper folder, and add the folder to the list pane in System Preferences. To add the folder, launch System Preferences, go to Desktop & Screensaver, and then click on the Desktop tab. In the pane that appears on the left side, click the Choose Folder option. Navigate to the folder that contains the holiday wallpaper, select it, and then click on the Choose button.
For Macs running OS X 10.5 or later versions, the Choose Folder item may be absent. If this is the case, click on the plus sign at the bottom of the list pane instead, and follow the same procedure. Once you have successfully added the folder, locate it on the list pane. Click on the folder, and then click on the holiday wallpaper you downloaded to set it as your desktop background.