That's a great reminder that the internet is full of cowardly bullies. They don't actually want to understand things, they just want to tear other people down to make themselves feel better.
A positive outcome of this was that these people dialed down their rage when the person in the photo commented. Before that however, they were still perfectly willing to expend a lot of angry energy trying to eviscerate the person they had created in their minds.
Hmm... I like how you center on "don't want to understand things". It's a LOT harder to hate something you understand, especially when it comes to people.
I'd quibble on the final part though: they do it not to make themselves feel better (that is simply not possible), but to justify feeling bad about themselves and not wanting to admit it. It's quite narcissistic.
I think it's also just a product of the hivemind and people not thinking. Not realizing that there is a real person in that picture. Causes normal people to be asses without thinking. As evidenced by the fact that the tone of the conversation changed when he responded.