I agree with you, usability is important. But as you said “here isn’t any difference between adding link into the img tag or into the shortcode” the first option actually teaches the user something. He can use that in any html document not only your theme.
If you noticed in my first post I even noticed that I avoid making customers use html but that’s not always possible.
If I handled refunds I would gladly refund him just for that. He started to insult me and call my work “sh*t” and I don’t want to work with rude clients.