Is AUTHOR wrong in the post ? POST : So this happened years ago when I was 18 and she was 17. (I'm 22 now)

I found out she showed another guy nude pictures of herself when I wanted to prank a coworker of hers by pretending to be her on her phone. Except there was a conversation above about how he appreciated that she showed him nude pictures of herself (in person), and she was like "No problem, I'm an open book". This was while we were in a relationship.

Obviously I was upset by this so I confronted her. She didn't say anything and just looked like a deer in the headlights she began to cry and I stormed out and drove home. I got a couple messages from her friends telling me how awful I am for walking out on her because "It was only 3 fucking photos!" and that it was apparently an *accident*. There's no way in hell I was believing that. These messages ramped me up even more.

I decided to message friends and group chats to tell them the news. In one group chat I said something like "Welp, now that that relationship is over.. anybody want nudes?", "I'll sell them for a dollar" and shit like that. (This is the AITA part).
I wasn't actually going to do it. It was just a heat of the moment joke.

The main reason I felt this was acceptable was because I wanted to make it bite her in the ass for being so "open". Kinda like a "How do you like it now?" type of thing. I didn't end up showing anybody any nudes.

This was a *massive* group chat with a bunch of people from our high school and a girl who isn't friends with either of us reached out to my ex and told her what I said. This girl was on my ex's side and turned a bunch of people against me. They even emailed my mom telling her what I said.

AITA for what I said? Who's the bigger asshole here? 

TL;DR
Ex showed nude pictures of herself to coworker, I said on highschool groupchat "Anybody want nudes of her?". Didn't actually send nudes, and got hate from pretty much everyone except my friends.