Doing it without their permission and posting it below their comments is definitely misogyny and sexual harassment. If you did that in the workplace you'd be fired for cause.
Fantasy is always consensual; the line is only crossed when people share these images. It's the difference between fantasizing about a co-worker and telling them about those fantasies.
The problem is, this service very publicly shares those images, and usually in a way that directly tags the actual person. So, very much across the line in to "harming actual people."
Since this regularly harms women, people round it off to "misogyny". Even if the motive is not "harm women", the end result is still "women were harmed".
(There also IS an exception if you know the person well enough to expect that they're cool with it - but the onus is still on you if you guess wrong and your actions end up causing harm.)
Misogyny doesn't just mean you hate women. It can also mean you believe them lesser, or you do not respect women, or you are actively trying to undermine them.
Sharing lewd pictures is using the tools of the patriarchy to shame and humiliate women. That's misogyny.
Think of it this way. I want to humiliate a black man online, so I generate a picture of him eating a huge watermelon slice and share it around for giggles. Is that racism? Of course it is.
