@rudolf I see what you mean. To save on mobile data I presume?
Mastodon's settings enable you to always hide media unless you click on it, so the idea already exists. But I guess you might want to keep your desktop environment and mobile environment settings seperate?
I'd call that 'an exception', really. But you can mimic this already with the mobile web interface. Although you wouldn't see the alt text so I suppose that's a plus for Tusky there.