Chastising people you don't know for not using alt text is the new accosting people using the priority seats on the bus that you don't think look disabled enough.
You cannot tell in what way someone else may be disabled just by looking, and that's true online as well as in meat space.
You cannot judge what is 'easy' for someone else based on what's easy for you.
Real allies don't harass one group of disabled people in the name of another.