A Muhammad Ali wallpaper. Available in 2560x1440px, 1920x1200px, 1680x1050px, 1440x900px, 1280x1024px, 1280x800px, 1024x768px, iPad, iPad with Retina display, iPhone 4, iPhone 5.
Helpful hint: Dribbble users are nice. If the license is unknown, it is usually because they forgot to mention it explicitly; not because they were being stingy. Ask nicely and they will probably allow you to use it.