When guys post photos on dating sites of themselves holding a fish they caught, they understand perfectly that women do not want to see that. Perhaps when they were very young, they thought a woman might be impressed that they could catch fish, and that adding some sort of primal "provider" photo would lure the ladies in. That it would show his strength and resourcefulness.
Since then, however, he has learned that only other men are impressed by the fish. The women, by contrast, understand that a man holding a fish in a dating profile photo would potentially be a husband who escapes onto a boat to avoid responsibilities. He would then claim that this is actually for your own good, because now you get to eviscerate, scale, and cook the fish for him.
Still, it's there for a reason for the "man holding a fish" photo. Some part of his mind is still bound by the hope that somewhere out there is a lady who will accept him for being a man who is going to go fishing to avoid her company. That somewhere there is a beautiful blonde in waders going flyfishing, who is looking for a silent, avoidant man, and who will keep her trap shut.
So you can tell him the photo won't attract ladies, but don't tell him to take it down. The red flag for you is honest advertising for him, plus he's still allowed to hope.