18 cards where the illustration contains “hidden-picture”

Cave of the Frost Dragon (Adventures in the Forgotten Realms #253)
Court of Locthwain (Wilds of Eldraine Commander #31)
Den of the Bugbear (Adventures in the Forgotten Realms #254)
Fetid Pools (Amonkhet #243)
Forest (Outlaws of Thunder Junction #276)
Forest (Unsanctioned #95)
Hive of the Eye Tyrant (Adventures in the Forgotten Realms #258)
Island (Amonkhet #260)
Island (Unsanctioned #89)
Lair of the Hydra (Adventures in the Forgotten Realms #259)
Mountain (Unsanctioned #93)
Painted Bluffs (Amonkhet #246)
Plains (Unsanctioned #87)
Swamp (Unsanctioned #91)
Sylvan Anthem (Modern Horizons 2 #176)
Sylvan Echoes (Lorwyn #237)
Winged Portent (Innistrad: Crimson Vow #89)
Yawgmoth's Vile Offering (March of the Machine Commander #271)

The message was clear to Sorin—time to search for something else.