How has browning presented the main theme of the poem “my last duchess”?

Browning has presented the main theme of the poem "My Last Duchess" through another piece of art - paintings. The Duke shows his guest the art found in his mansion, and stops in front of the painting of his ex wife, the Duchess. That is when the whole poem actually starts, as we get to know details about her and the plot/main theme develops. 

cause, what did the poor cats do to us.. and who can kill a cute loving animal!

Explanation: