The third picture is correct. The people in the first and third picture are pinching at the knee and pivoting off of that point which causes their leg to swing back.
You see this a lot in the Jumpers (and sometimes in the Hunters), where Equitation isn't scored. A lot of it has to do with height of the fence, because the fences are getting higher and higher, riders tend to get quicker with their bodies and try to help the horse jump the fence. (At least at lower levels then GP this is always the way it comes off)
At the GP level we can assume that the riders are so accomplished, that they have enough control everywhere that letting their legs slip back won't cause a complete catastrophe. And then again some GP riders just have awful equitation and aren't that accomplished of riders... lol