Is defiling a girl's honor by displaying her picture to others an unforgivable sin? And how can one atone for this sin without informing the girl concerned?

If a girl has had her honor violated by having her pictures publicized, there is no sin in showing her picture to another person.

However, if the picture incites temptation, or if the person to whom it is shown is not permissible to look at it, then there is a sin in that; because it is assisting in disobedience and spreading it. Allah Almighty said: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression" (Al-Ma'idah: 2). This is because "the one who points to evil is like its perpetrator."