Will the girl whose picture I kept – without her knowledge – and shared with a friend of mine – to know his opinion – take her rights from me on the Day of Judgment, despite my repentance and seeking forgiveness? And is it obligatory for me to inform her of what I did or not?

What appears is that you have wronged the girl by publishing her photos, especially since she does not want anyone to see her photos. Therefore, you are obligated to repent, delete the photo, and not keep it. The preponderant opinion is that it is not a condition for such moral rights to obtain absolution from their owners; rather, it is sufficient to supplicate for their well-being.