brief, full sized, randomised image shown on page load

brief, full sized, randomised image shown on page load

I need to fade in a an image (it will cover all content) of 100% height and 100% width for 3 seconds each time my index page loads, and then fade out. It also needs to be a randomised image (1 of 6).  I have been searching for over two hours and I am not having much luck. Can anyone point me in the right direction?