The speaker, the Duke, notes that when Fra Pandolf painted the portrait of his (Duke's) last duchess, he was present. The Duke felt he needed to be present because he wanted to control every aspect of the Duchess's life. He is jealous and controlling. He says:
Sir, ’twas notHer husband’s presence only, called that spotOf joy into the Duchess’ cheek;
Oh, sir, she smiled, no doubt,Whene’er I passed her; but who passed withoutMuch the same smile? This grew; I gave commands;Then all smiles stopped together.