"It appears that he...specialized in depicting street scenes, architectural views, and nocturnal occult subjects."
-Rembrandt, by Christopher Baker, Brompton 1993
The Sibyl showing Aeneas the Underworld
Breugel not enough for you? Swanenburg seemed to specialize in scenes of hell with a gaping maw at the lower right.
You could make a game out of catching the similarities and differences between the above and below:
Canadian death metal band Sympathy used a detail for their 2005 EP Abyssal Throne (well, their designer Samuel Durling did):
These aren't anything necessarily revelatory, but kind of fun:
The gate-demon in this one is kinda like Falcor gone very wrong:
The Final Judgement and Seven Deadly Sins
His The Fall of Troy is a departure:
Thinking about Seven Deadly Sins solidified something I've been thinking about regarding inventing fantasy religions, based on how I think some real religions formed.
If you have a history for your setting, or just some ideas for it, think about what things the authorities would have had trouble getting the populace to do in different regions, what behaviors they would have wanted to encourage or discourage in the populace. The religions that would promote that which those in authority want to encourage and make taboo that which the authorities wanted to discourage would have gotten a boost from those authorities, and be more dominant.
Wish I had more time to develop that, but it's a start.