What should you do if you suspect that you have taken pictures of someone, which is not permissible, and you cannot contact them to seek their forgiveness? Is it sufficient to supplicate for them?

As long as the enquirer is unsure whether he took it or not, the default is the acquittal of responsibility, and nothing is binding on him. Indeed, Al-Izz ibn Abdus-Salam and Al-Juwayni mentioned that the default for anything whose obligation is uncertain is the acquittal of responsibility.