The "KEEP OUT" part is simple enough. The crossing of the fingers is a little strange, but gets the idea across about the area being forbidden.

The real mystery: What is the white stuff on the left hand's finger? Why is it there?

My Hypothesis:

Of course, the first thing that comes to most of our minds when we see this "white stuff" is semen. Why would they incorporate semen into this picture? Well, would you really want to enter an area where the warning sign involves a hand with sperm on its finger? I most certainly wouldn't.