I got these pictures from A Cup of Jo, who got it from black eiffel, who got it from Waldo's Post. So they're well on their way to becoming viral.
They're pictures from Victoria Falls in Zimbabwe, where between September and December you can go to an area called The Devil's Swimming Pool and swim to the edge without being swept over! This is due to a natural rock wall just below the water at the very edge of the pool that you can see in the photographs and that slows the current even at the edge. Still - not for the faint of heart!