The orientation of that guy is wrong. Now, place his hands on the ball, and feet on a couch, then do the pushups.
Do you have a ball handy? How many can you crank out? And, this is ATOT we're talking about.
On fresh arms/chest I can easily crank out 20 in a row, but can definitely feel it as I approach 20. I've done these as part of a circuit and would do 3 sets of 20 - with the second and third set being exponentially harder and requiring breaks. I'm by no means some super athlete, but I can usually make the first set of most things look pretty simple. It's all downhill from there!
I've done pushups with a medicine ball as suggested above, too. I actually struggled more with that because my hands were closer together so it was more triceps with less help from chest. That, and I'm 6'2" with lanky arms so it was even more noticeable.
I'm currently working on single arm pushups, or as I like to call them, demotivators. I can do two on each arm and then am shaking like a little girl, haha.