Pictures can make your proof argument more impressive, but it's hard to beat a live, in-person demo. And if you can't do a live demo - try video. Take a look.
Can't you just hear this conversation on some jobsite?
Tough guy iron worker: "I don't need a hard hat. It'll just get in the way, I'll take it off & lose it. It can't really make that big of a difference. And besides - I've been doing this kind of work since before you were born ... and nobody's hit me with anything yet."
How do you argue with someone who has an over-abundance of confidence in what they believe?
It's hard. I guess if you were their boss you just tell them to "Just do it!" But then you really haven't convinced them have you?
Years ago back in Iowa I helped run a Hunter Safety education class for kids. We could tell them things, but it wasn't until we actually showed them what we were talking about that it hit home. We did lots of demos.
For one demo we'd show them how not to assume just because you set the safety on a gun that it won't fire. We had a gun with a worn out safety that we would bring in to one of the eight classes. It would fire a round in the chamber if you banged it on your knee (blank round of course). You should have seen their eyes grow as big as saucers when it went off. Ask them if they were crossing a fence and dropped that gun by accident, what might happen to one of their hunting partners, and every kid paid attention.
We also did a demo similar to the one you see above. Turns out science says a coconut has more similar characteristics to the human head than a watermelon. But if you're looking for visual impact in a demo, a watermelon works well. Show a kid what accidentally discharging a shotgun could do to a watermelon head, and you've made your point.
I ran across the video above a few weeks ago. I think it's pretty effective.
In fact, I think it might be effective enough to make a point with any halfway intelligent tough guy. It's hard to argue gravity.