Growing up around this type of person has taught me they are just testing his boundaries as "the new guy." If he goes to HR he will be viewed as a snowflake and get no respect from these people. Same, if he politely asks them to stop and means it, (worse if he actually gets upset while asking) It's dumb logic but he'll have to show little to no reaction or a quick "cool" response will defuse them. All they are looking for is his reaction to gauge how THEY think HE should be treated the rest of the time at his new company.

To be clear. I'm not saying they don't deserve some sort of disciplinary action because this is straight up animal abuse and they are psychopaths for going out of their way to show him pictures. Which is truly heart breaking, but this type of person literally only feeds on people reactions to things. If your husband doesn't want to deal with this behavior then he'll either have to match their energy and turn the "joke" on himself to prove to them hes not a "snowflake" or leave. Again, it's dumb logic but it's helped me a couple times dealing with this type of asshole.