That's two different things. I think if it's in the public (as in, on the street, in a public place, etc...), there is by definition nothing wrong with looking. A woman in a crop top, by definition, is showing off her belly; shouldn't be surprised that it attracts our gaze.
Now pictures, and sharing them on the Internet... On the one hand, when in public, one is allowed to snap pictures, after all. I've done it countless times; we take selfies, pictures of monuments, pictures of our food, pictures of where we are... And there's other, anonymous people who appear in the background. It's sort of a grey area, IMO
Then there is the unambiguous case of the breach of trust. When you start sharing intimate pictures, shared originally between only two people in absolute confidence to end up for all to see. That is disgusting, hurtful, and should be firmly opposed whenever possible.
I knew a guy back in my student years, a corporate lawyer. Very smart, I liked to hang out with him. Until one day he whipped out his phone and started to show us intimate pictures of his past conquests, asking insistently "Aren't they hot?" or sharing comments like: "This one's kinky", "This one likes to take it up the ass" and other disgraceful observations. Since that day, I don't hang out with him anymore; this is the mark of a man who cannot be trusted.