A Portrait Ghost is a ghost that was once a portrait in the game Luigi's Mansion.
According to Professor E. Gadd, the portrait ghosts were released from their paintings by Boos under the command of King Boo. Luigi had to vacuum up all of the portrait ghosts (except for a few "optional ghosts" like Mr. Luggs) using the Poltergust 3000. Captured portrait ghosts would be returned to their paintings via the Ghost Portrificationizer. The frame and the style of the portraits would change depending on how many pearls Luigi acquired during the battle with the ghosts.
Black and white pictures can be found in many of the Portrait Ghosts' rooms in the mansion. These pictures look similar to each ghost that inhabits the room they are in, suggesting these pictures may be depicting these ghosts when they were alive. Pictures of Chauncey, Slim Bankshot, Henry and Orville, and Sue Pea can be seen in the Nursery, Billiards Room, Twins' Room, and Guest Room, respectively.
List of Portait Ghosts
Note: Although Luigi first meets Madame Clairvoya in Area Two, he captures her after he begins Area Three.