This is bonkers. The attribution is still there, you just have click it. Besides, I doubt anyone who cared about the image is going to be content looking at the low-res thumbnail. The reputational damage done to the photographer is negligible.
Punishing the retweeters is also problematic because they themselves didn't crop the picture, twitter did. What does this mean for cropping that's done by the user-agent, or is otherwise device specific? eg. you post on the desktop site, where nothing is cropped, so you think everything is okay. Little do you know on the mobile site, all the images get cropped. Are you now liable as well?