Brent makes a good point.

Additionally, looking at the pic in question, another thing comes to mind:
It's quite possible for your airplane to jump the chocks if you run the
engine up to anything more than just a bit. If that happens, and no one is
at the controls, the results could be nasty. Even with the controls manned,
it easily could catch the person by surprise and do some damage before he
can react - like running into the person standing in front of the wing.

Better than just using chocks, it's common practice (at least where I did my
first run-ups) to tie the mains to some sort of anchor - like a car or
whatever - with a couple of lengths of stout rope.

Just a thought.

Dan Schaefer