What is the expiation for sending a picture of a girl to a young man without her knowledge, given the impossibility of asking for her forgiveness, and is supplicating for her in her absence sufficient?

You have done well by regretting, repenting, and cutting ties with the young man. You must be careful not to return to such actions in the future. The completion of repentance concerning the rights of people is by seeking their forgiveness. However, some scholars have chosen the opinion that for such intangible rights, supplication and seeking forgiveness are sufficient.