How is power presented in "My Last Duchess"?

"My Last Duchess" is about the power that the speaker, a Duke, had and still has over his dead wife, the eponymous Duchess.
The Duke keeps a painting of the Duchess behind a curtain which "none puts by" but him. In this way, the Duke has power over the Duchess in that he controls who now sees her and what stories people hear about her. He implies to his guest that the "glance" in the Duchess's expression, and the "spot / Of joy in the Duchess's cheek" are signals of her infidelity. The Duchess, of course, has no opportunity to defend herself or to put forward her own story.
Later in the poem, the Duke boasts that he "gave commands; / Then all smiles stopped together." The implication here is that the Duke, jealous of the Duchess's smiles and assuming that they were a sign of her infidelity, "stopped" them by taking her life. Thus, he had then the power to end her life, and he has now the power to control how, or if, she is remembered.
At the end of the poem, the speaker points to a bronze statue of Neptune "taming a sea-horse." This statue can be read as a symbol of the Duke's power over the Duchess. The Duke is represented by Neptune, the Roman god of the sea, and the Duchess is represented by the sea-horse. The difference in size between the god and the sea-horse represents the imbalance of power in their relationship. And the fact that Neptune is "taming" the sea-horse implies that this is what the Duke, at least from his perspective, used his power to do. He tamed the Duchess who he thought too wild and too independent.