Fat kids can't run! Their thighs rub together which if sustained can cause the child to spontaneously combust and as you can imagine with all that fat packed on - the little fucker will go up and stay lit for quite some time. Probably long enough for a homeless ex-banker and his family to gather around for the night to stay warm.
No, I'm guessing this is an image of the fat kid falling on his face. Which is always a sad, pathetic, hilarious sight to see.