The Official site has introduced a download section for Icons, Wallpapers, Posters, Screensavers , Widgets, Avatars, Mobile Ringtones, etc.
Till the games arrives you must have a look at the Official Wallpapers of GTA IV released by Rockstar.
There are 20 wallpapers available for Free in many sizes given below:
Widescreen: 2560×1600 , 1920×1200 , 1680×1050 , 1280×800
Standard: 1600×1200 , 1280×1024 , 1024×768 , 800×600
Handheld: Blackberry , PSP , iPhone