The first twenty-one lines of the poem are quite detailed about the portrait. Firstly, we learn that the painting is of the speaker's last wife and that the portrait is quite vivid: Looking as if she were alive. The duchess is obviously deceased. The speaker (the duke) says that he calls that piece a wonder which suggests that the painting had been skilfully executed and that the artist's mastery resulted in a beautiful and remarkable image of the duke's wife.
We furthermore know that the artist, Fra Pandolf, had taken a day to finish the portrait and that it is a full-length painting - and there she stands. The duke courteously invites his guest to sit down and take a proper look at the picture. He is obviously proud of having been married to such a wondrous woman.
It is clear that the artist had quite vividly captured the blush in the duchess' cheeks. The duke, displaying his vanity, refers to himself in the third person:
... Sir, ‘t was not
Her husband’s presence only, called that spot
Of joy into the Duchess’ cheek
The duke concedes that the remarks made by the artist during the sitting could also have made his lady blush. It becomes clear that the duke was not much pleased by her reaction to such courtesy, for the following lines introduce a quite sinister element:
... such stuff
Was courtesy, she thought, and cause enough
For calling up that spot of joy.
As we continue reading, we become alarmed at what the duke has to say and suspect that he, in his arrogance and vanity, may have done away with his wife, suggested by: Then all smiles stopped together.