Even in the Regency, marketing plans were useful, especially when parents were seeking to marry a daughter advantageously. Lord and Lady Curran expect their daughter's portrait to convince potential suitors of her beauty, her worth and her desirability. Of course, it must also show her as a perfect, obedient, demure lady.
Kermit Sutherland is a popular portraitist, so of course he is engaged to produce the portrait. What Chastity's parents don't understand is that Sutherland paints more than the surface. He has a knack for seeing into a woman's heart and soul.
Under her obedient façade, Chastity harbors a rebellious heart, and Sutherland sees it and encourages it. When her portrait is finished, it might show more than her parents--or she--have bargained for.