I am not a halachic authority so I will only respond with my personal feelings. It’s very wrong if the purpose is gain monetary benefit. It’s just as wrong if the purpose is to make prospective students think that the yeshiva is different than what it actually is. It’s wrong even if the purpose is just to improve the yeshiva’s PR, because whatever the reason, a picture is intended to convey a visual fact that’s there when the picture was taken. By altering the picture you are essentially creating a picture that lies. Do that in a court of law without indicating that it was altered and you can go to jail.