Charles Wilson Peale painted the portrait in Annapolis in 1771, depicting Hallam not in Shakespeare's play, but in a musical adaptation based upon it. Hallam was especially known for her singing, and performed widely before the outbreak of the American Revolution.
Unfortunately, the war proved devastating for acting companies. Hallam, who had played roles in The Clandestine Marriage, Love in a Village, The Provoked Husband, and other plays, had to leave with the rest of her company to Jamaica in 1774 as things were heating up in the colonies. She ended up staying there and marrying a church organist.
Hallam was praised for her "classical strictness of expression" as well as her beautiful voice. Her expression is certainly noteworthy in the painting, but more striking is her outfit. Imogen is supposed to be dressed as a boy in the scene, but as for why a British damsel should be dressed in an oriental costume for a trip to Wales, your guess is as good as mine.
That Peal chose to paint a heroine with sword drawn escaping from tyranny might have been a comment on the political situation before the revolution. In any case, few people today remember Hallam's career outside of this portrait.