How can a woman repent to Allah after regretting sending pictures of herself to her fiancé before marriage, while she was bareheaded?

If the purpose of sending your picture to your suitor is for a lawful viewing (Ru'yah Shar'iyyah), then we hope that no sin will befall you for that. The opinion that restricts viewing to only the face and hands is not a matter of consensus; rather, the opinion permitting the viewing of the feet and neck is strong. However, if the picture is sent after the viewing has already taken place, then it is not permissible.