High voltage and electric shock
I have read that the resistance of the human skin is 100,000 ohms, which means the current through the body would be 240V/100,000ohms = 2.4 mA . 


That's mostly true, but if your skin is damp, or worse, damp and salty, its  resistance will be lower.


  I also read that lethal current through the body is around 15 mA, so 2.5 mA quite low?


Also mostly true, but if you have a heart condition, or wear a pacemaker, or just get unlucky, the lethal current for you could be lower than the average one.


  I am insulated from ground (for example standing on a chair), what would happen if I shove a fork in to the power outlet (or some other metal so that the power is short circuited).


Typically this will start to melt and or vaporize the shorting wire, with a fairly dramatic show of light and heat. 

A small current will also flow through your body due to the capacitance between you and the ground.

Usually you're on your butt before you realize what's happened. (Note: I've only encountered this with 110 V mains. Don't try it at home)


  In the first Image the person is connected to ground (0) and is also holding the the high voltage line (240V), so the electrons/current would want to flow through him i would assume?


Notice that in the first picture, the power source is not connected to ground, so the unlucky guy's foot connected to ground doesn't produce any path for current to flow back to the power source.

In the second picture, both the guy and the power source are (indirectly) connected to ground, so there is a complete circuit formed through the guy and the ground, and the guy is not going to have a good day.