The graphics on a web page can be protected by copyright.  If they are distinctive enough, they can also be protected as a trademark, and even registered.  However, a third alternative, design patent, which can protect such things as website backgrounds and other website features.  See, for example,  U.S. Patent No. D570,362 on the background image for a portion of a display screen:

Google obtained U.S. Patent No. D599,372 on its landing page: