Hey @dani38! At this time, alt text can only be added to form & landing page imagery via the image edit modal that is revealed by clicking on the image.
However, for images that are added in the default image placeholders, this modal is unfortunately not accessible at this time.
In other words, alt text can currently only be added to extra images added to your opt-ins (more info on how to do that here83) — once they're added, you can click them to reveal the edit modal, which includes a space for alt text:
I'm sorry about the limitation there! I'd highly recommend filing the ability to add alt text to all opt-in images as a feature request for our team here42 :)
(Also, just created/added the accessibility tag to your post for you!)