[beardog]

Imagine you were distributing my work without permission and in retaliation I emailed you gross-out sexually explicit imagery. That would both be a crime and be unethical by current standards.

OP is not emailing people, but it is the same thing: revenge using explicit imagery. Except not only might the people behind the sites see the image but their visitors which probably include minors will too.

[datadrivenangel]

Better example is if someone is automatically forwarding your email newsletter and claiming it as their own.

If you email them explicit imagery and they automatically forward it, that's on them.

Not a crime, but a little crass in a funny way.