Some cards just came from googling the title plus castle. So, Cellar is some castle's cellar; chapel is some castle's chapel. A few cards are SCA pictures, like Trading Post. Some are ads for costumes, like Ambassador, or, in the case of Feast, ads for props. And then there's just a lot of random fantasy art or what have you. I do google image searches, but also look on deviantArt and Epilogue.
Bureaucrat, Militia, Spy, Festival, Embargo, Cutpurse, and Cartographer are from Magic cards.
Witch is from the Wizard of Oz.
Adventurer is some Conan art.
Pawn and Duchess are from Alice in Wonderland.
Swindler is Reynard the Fox.
Minion is Wormtongue from Lord of the Rings.
Saboteur and City are from computer games.
Upgrade shows work being done on Castle Neuschwanstein.
Haven is a painting of the place used in the opening scenes of Fantasy Island.
Lighthouse is the Lighthouse of Alexandria.
Smugglers I cropped from a novel cover.
Warehouse is from Raiders of the Lost Ark.
Pirate Ship is a Peter Pan painting.
Sea Hag is from Popeye.
Outpost is the castle of Saladin, on Pharaoh's Island in the Red Sea.
Tactician is Napoleon.
Apprentice is from Fantasia.
Trade Route is a random Silk Road picture.
Monument is the Colossus of Rhodes.
Counting House is an illustration for Sing a Song of Sixpence.
Jester is Danny Kaye from The Court Jester.
Followers is from Monty Python's Holy Grail. Neil Innes in front, who wrote/sings about Brave Sir Robin.
Princess is some Snow White thing not actually from a movie or anything.
Crossroads is the only pic I modified. I erased some telephone poles. It's the crossroads where blues singers go to sell their souls to the devil.
Fool's Gold does too show pyrite.
Oracle is some painting of the Oracle at Delphi.
Scheme is from Princess Bride. There appears to be a big overlap between Dominion playtesters and Princess Bride fans.
Jack of all Trades is a Jack!
Noble Brigand is Douglas Fairbanks as Robin Hood.
Haggler is from Life of Brian.
Highway shows the Appian Way - one of those roads that lead to Rome that you hear about.
Stables is at Kenilworth Castle.
Walled Village shows Carcassonne's walled village. It's not just the name of a game.
A few cards on isotropic don't match the prototype, because the prototype had the actual art (or did at that point). Doug came up with his own images for some of those. This includes Goons, Stash, and Black Market.