The Fresco and False Accusations
The painting technique becomes crucial evidence of the Duke's calculated cruelty. He chose a fresco (painted directly onto wet plaster), which must be completed before the plaster dries, creating artificial urgency.
This reveals something horrifying - he didn't decide to commission her portrait until after he'd decided to kill her. The artwork matters more to him than his wife's life, showing his callous priorities and how quickly he wanted her executed.
Browning's audience would recognise that Fra Pandolf is a monk, making the Duke's jealousy accusations even more ridiculous. He admits he was present during the painting sessions, so there's no possibility of an affair - yet he still finds reasons to be suspicious.
The phrase "spot of joy" deliberately echoes Lady Macbeth's guilt, suggesting the Duchess's only "sin" was finding happiness in life - something the Duke himself never experiences.
Remember: The Duke only takes pleasure in objects he can own (art, money) - never in human relationships or simple joys.