What Dominion cards have artwork featuring different versions of the same object?
The vast majority of the cards feature unique artwork, which is great. However, there are some cards that indeed include other cards' artwork, like:


Scrying Pool (from Dominion: Alchemy) depicts the artwork from Village. Just turn the card upside-down to see the reflection in the pool.
Minion (from Dominion: Intrigue) displays a  portrait of a Minion. Self-reference.
If you put Island, Pirate Ship and Native Village (from Dominion: Seaside) next to each other, in this order, you shall notice that their artworks make up a greater panorama.
On the same token, most Split Piles from Dominion: Empires and the Sauna/Avanto promos feature continuous artwork.
Likewise, the two traveler lines, Page and Peasant (from Dominion: Adventures), illustrate the same person in evolving stages of their lives. (added by @Gendolkari)
Candlestick Maker (from Dominion: Guilds) is crafting the candle depicted on Masterpiece, from the same expansion.
As mentioned, Pillage (from Dominion: Dark Ages) depicts a Village under attack.
As mentioned, most Ruins depict destroyed versions of other cards: Ruined Village/Mine/Library/Market depict Village/Mine/Library/Grand Market respectively.
Overgrown Estate (Shelter from Dominion: Dark Ages) lives up to its name by depicting an abandoned version of the Estate artwork, as illustrated in the second version of the base game and in Dominion: Base Cards. (added by @Ned Strong)


Cards that pay homage to other games:


City (from Dominion: Prosperity) displays the box art from the game Carcassonne: The City.
Governor (Promo) displays the Governor role art from Puerto Rico: Limited Anniversary Edition. Its card symbol, a ship, is the same ship from the box art. 
The Walled Village (Promo) card's symbol is a meeple from Carcassonne. The card itself supposedly depicts Carcassonne, as well.