Currently, you can't opt out from this at all. But here's few things you can do as a protest.

FOR SERVERS YOU ARE AN ADMIN IN

Go to Roles -> @/everyone roles -> Scroll all the way down to External Apps, and disable it. This won't delete the option, but it will make people receive a private message instead when they use it, protecting your users:

You should also make it a bannable offense to edit other user's images with AI. Here's how I worded it in my server, feel free to copypaste:

Do not modify other people's images with AI under ANY circumstances, such as with the Discord "enhancement" features, amidst others. This is a bannable offense.

COMPLAIN TO DISCORD

There's few ways to go around this. First, you can go to https://support.discord.com/hc/en-us/requests/new , select Help and Support -> Feedback/New Feature Request, and write your message, as seen in the screenshot below.

For the message, here's some points you can bring up:

Concerns about harassment (such as people using this feature to bully others)

Concerns about privacy (concerns on how External Apps may break privacy or handle the data in the images, and how it may break some legislations, such as GDPR)

Concerns about how this may impact minors (these features could be used with pictures of irl minors shared in servers, for deeply nefarious purposes)

BE VERY CLEAR about "I will refuse to buy Nitro and will cancel my subscription if this feature remains as it is", since they only care about fucking money

Word them as you'd like, add onto them as you need. They sometimes filter messages that are copypasted templates, so finding ways to word them on your own is helpful.

ADDING: You WILL NEED to reply to the mail you receive afterwards for the message to get sent to an actual human! Otherwise it won't reach anyone

UNSUSCRIBE FROM NITRO

This is what they care about the most. Unsuscribe from Nitro. Tell them why you unsuscribed on the way out. DO NOT GIVE THEM MONEY. They're a company. They take actions for profit. If these actions do not get them profit, they will need to backtrack. Mass-unsuscribing from WOTC's DnD beyond forced them to back down with the OGL, this works.

LEAVE A ONE-STAR REVIEW ON THE APP

This impacts their visibility on the App store. Write why are you leaving the one-star review too.

_

Regardless of your stance on AI, I think we can agree that having no way for users to opt out of these pictures is deeply concerning, specially when Discord is often used to share selfies. It's also a good time to remember internet privacy and safety- Maybe don't post your photos in big open public servers, if you don't want to risk people doing edits or modifications of them with AI (or any other way). Once it's posted, it's out of your control.