Seriously, don't worry about it at all - just post them up and if your game's any good you'll create some buzz and have plenty of time to swap stuff out later. They'll create much more buzz with some half decent pics in there as opposed to being totally ignored when stuffed full of abysmal '80s clip art.
No. definitely worry about it. The last thing you want is to crash and burn your reputation as a designer by using artwork or photos without permission. Whatever buzz you might enjoy will be negated by the infamy of your career's death.