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