Illegal Sexual harassment is unwanted contact of a sexual nature in the workplace. It appears that up until you sent a picture outside of work, everything that occurred was wanted by her. You evidently, if not inadvertently crossed a line, but since it was off work site and not during work hours, there could be no workplace sexual harassment. Plus, as co-workers, one text message or photo after work would not be seen as severe or pervasive to be actionable.
Tthe employer has a duty to immediately investigate the complaint. If you can prove the conduct occurred off work site after hours then you would be benefited by doing so and explaining what occurred was invited and not in the workplace, and that you are now cutting off contact. Obviously, this person cannot be trusted and you would be wise to cease and desist from any further contact with her and to document and complain if she contacts you again. Sexual harassment complaints are taken seriously and often badly by the employer so you need to do what you can to protect yourself by documenting exactly what ocurred, how it was consenusal, how it occurred off site, and how it will not happen agains.