Are the moderators sinful for not deleting subscribers who refuse to change their inappropriate profile pictures?

You must delete the forbidden pictures, acting upon the evidence indicating the changing of evil and its removal by whoever is able to do so, as the Prophet, peace and blessings be upon him, said: "Whoever among you sees an evil, let him change it with his hand." This is an obligatory matter by the consensus of the Ummah. If you are able to remove and delete those pictures, then you must do so, along with advising their owners gently and wisely.