Is designing a large advertising image that includes a stone statue in the form of a human, based on the manager's insistence, considered a sin that affects one's faith? And what should the designer do about it?

Statues of living beings are forbidden by scholarly consensus. Photographing them and hanging them up constitutes an affirmation and propagation of a reprehensible act, and is therefore impermissible. If you are unable to remove the picture, you are not obligated to do so; your repentance and remorse for what has passed are sufficient for you.