Is the sender of pictures of an unveiled girl to an unknown person burdened with the sins of everyone who views these pictures if the latter publishes them? And what is the way to repent from this sin?

If you can prevent your friend from publishing the photo, then do so. Otherwise, his publishing of it will not harm you, nor will you bear the sin of those who see it. This is because whoever repents from a sin, his repentance is accepted, and he is not harmed by the persistence of an effect of the sin that he is unable to remove.