Greatest knees online...
...can be found here. They are happily displayed in their natural habitat.
However, since we cannot avoid showing some of these knees' "usual surroundings", we urge you to double-check your age before clicking on and viewing our pictures. This site is meant for people over 16 and most pictures can only be seen by members. (Membership is of course free.)
All that said: we still need lots more material. Why not sign up and show off your own knees? Upload a couple of nice pictures and be admired. Look at the stars to see if people have rated your photo already. If they haven't, don't hesitate to ask them to. And please, don't forget to rate other people's photos yourself.
PS: you don't need to include your face in your pictures, but you can if you really want to.