A portrait oil painting is an item in Evony TKR (The King's Return).
Portrait Oil Painting is one of the collection items in the exhibition for the Art Hall. It is one of four parts in the Oil Painting Hall. After the set is completed at three stars, you can trade additional Portrait Oil Painting fragments for gold.
The description of Portrait Oil Painting in Evony TKR says:
Hand painted works by the masters. The important exhibits in Oil Painting Hall.
Portrait Oil Painting is known to potentially be obtainable from opening Artwork Fragment Chest II, Artwork Fragment Chest III, Artwork Fragment Chest IV and Artwork Fragment Chest V. The links for each item are below:
Note: This list may be incomplete.