How can one atone for the sin of uploading pornography to the internet, which has been seen by millions, after repentance and drawing closer to Allah?

If a person repents sincerely from a sin, it is as if he never sinned, and he is not harmed by many people having witnessed his wrongdoings. He should remove the traces of the sin if he is able, increase in obliterating good deeds, and strive to spread goodness and invite to it.