Edits of an image don't really seem like that horrible of a thing. It seems like he wants to get mad at it.
I mean for forgets sake the guy just flipped the mouth upside-down. That's literally nothing
it really isn't up to you if it's offensive or not. you don't know what it's like for people to act like they have the right to make fun of you by editing your own art and laughing at it's flaws.
giving criticism on a person's flaws is different from straight out ridiculing them. they are doing the latter.
they are doing that stuff specifically to annoy flat and they've been at it for a long time. at this point you don't get to decide how flat should react because the ones responsible for making him feel that way are them. considering someone's feelings towards one of your actions is what being a friend or even a decent person is all about.