It's probably a mix of throwing shit at the fan, power dynamics, the thrill of the taboo and probably a little bit of a humiliation fetish. It's never about the receiver. It's an aggressive act.

A picture is worth a thousand words

"Hi I am...I am looking for sex"

If you are using a dating website and actually trying. You'd look at a profile, read their bio, and then send a message that had something to do with their bio. (Could also comment on pictures)

But it is a numbers game to an extent and most messages sent will never see a reply.

So what is the least effort highest reward thing to send first? Private picture.

They are hoping that the person opens it and it catches their eye.

If their first thought is to send a private picture, they are not really looking for small talk. So it has the added bonus of weeding out non-sexual people.

It's no risk, high reward

Or they just think if they see tits it makes them horny so women MUST be the same way?

This one. The problem is that by the time 90% of the hetro cis male population of one generation has figured this out, a new generation of teen hetro cis males are right behind them. A better sex education class would cover this.

the principle equivalent genital exchange.

they think if dick pic is send vag/boob pic will be the response

I've never sent one, because anxiety n stuff. But from talking to the few of my friends that do it works quite often on tinder. They basically treat tinder and and instant hookup app. The flow is basically swipe > wanna hook up here's what i got? > ok come over.

My friend says works 100% of the time on fat women and 15% of the time on the rest. Which is enough to always have a few girls to call over at any time.