A small png. Download one of those things, they're five times the size of what gets displayed, but you have to download the whole thing anyway. A little responsibility would have been good.
"But then it's not pristine" -- sure, but also is a lot less pretentious and more enjoyable to visit. It's just simplicistic design... I mean, nothing against painting a blue square one a red canvas and calling it "Mothership 20B" or something, but that's not a Van Gogh. You can hang it in an art gallery if that is what makes you happy.
(I actually find many of them quite pretty, I'm just being mean because some folks could really use a dose of reality - form following functionality is not a new concept, neither is minimalism, and "flat design" is not something I will ever say with a straight face, I utterly refuse to pretend there is now suddenly such a thing. As the saying goes, some day we will look back on this, laugh nervously, and change the subject.)