I see that there's a trivia section for each card, so here is all of the art trivia I can think of.
Bureaucrat: The art depicts the artist, Matthias Catrein.
Harem: The art depicts developer Valerie Putman. She's in the back, between the giant half face and the man.
Native Village: The art connects with Pirate Ship to the left.
Pearl Diver: The art depicts playtester Wei-Hwa Huang. He won a tournament at the Gathering of Friends where being depicted was the prize.
Island: The art connects with Pirate Ship to the right.
Navigator: The art depicts developer Dale Yu.
Pirate Ship: The art connects with Island to the left and Native Village to the right.
Bazaar: This art was turned in for Market, but by mistake another artist was also doing Market.
Scrying Pool: Village can be seen in the scrying pool.
City: This art was also used for the box for Carcassonne: The City.
Counting House: The art shows the victory point shield on the wall.
Goons: This art was turned in for Pawn.
Duchess: The art is related to the art for Duke.
Abandoned Mine: The art shows a ruined version of the art for Mine.
Mercenary: The art includes a dagger and red sash from Urchin.
Ruined Library: The art shows a ruined version of the art for Library.
Ruined Market: The art shows a ruined version of the art for Grand Market.
Ruined Village: The art shows a ruined version of the art for Walled Village.
Overgrown Estate: The art shows an overgrown version of the art for Estate, from Base Cards.
Storeroom: This art was turned in as an alternate option for Vault.
Sir Martin, Dame Anna, Dame Josephine, Sir Destry, Sir Michael, Sir Vander: The art depicts a playtester.
Pillage: The art shows Village, under attack.
Black Market: The expansion symbol is the Spielbox magazine symbol.
Envoy: The expansion symbol is the Wiener Riesenrad in Vienna.
Walled Village: The art shows the medieval village Carcassonne. The expansion symbol is a Carcassonne meeple.
Governor: The art continues art used for Puerto Rico: Anniversary Edition; the expansion symbol is from its cover.
Stash: The art includes faces of SdJ jury members on the coins. The expansion symbol is the award poppel.