I was thinking about why the defense of "he does this all the time with students, look at these other examples of pictures with mildly insulting, animal-related captions" didn't sit right with me, and then I realized it's because the kids are the butt of the joke. That might have been fine in the past, but we know better now. Those aren't pictures of kids having fun at the zoo interacting with animals, they're posed pictures the teacher later gives captions comparing them to animals in a denigrating way (like calling them ugly, and lets not forget that "monkey see, monkey do" is supposed to be an insult - usually indicating that a person is just mimicking something without being able to understand it). That just doesn't seem like a thing a teacher should be doing to me. They're supposed to be supportive, not insulting, and having your own fun at the kids' expense seems like it crosses a line IMO. Granted, my only experience working with kids was a few years as a camp counselor, but I can't imagine anything like that flying there. Or even any of the counselors thinking it was a good idea. We were always encouraged to take our goofy pictures having fun with the kids.
For the people trying to act like this would have been harmless with white kids, I'm going to have to strongly disagree. No matter what, they pictures would have been inappropriate, the teacher's carelessness just made that obvious by giving those insults a racial dimension as well.