A Portrait Ghost (also known as a Gallery Ghost) is generally a ghost that was once alive, though at least four leave that up for debate. There are a total of 23 of these in the game (five of which are optional). They appear in Luigi's Mansion. According to Professor E. Gadd, the portrait ghosts were released from their paintings by Boolossus under the command of King Boo. Luigi had to vacuum up all of the portrait ghosts (except for the "optional ghosts" Mr. Luggs, Biff Atlas, Slim Bankshot, Jarvis and Sue Pea). Captured portrait ghosts would be returned to their portraits via the Portrificationizer.
The frame and the style of the portraits would change depending on how well Luigi performed in the battle. His performance would be rated by what kind of Pearls he got, which is determined by how much health of the ghost he drained in one suction. To get the big pearl, and consequently the gold frame and portrait, Luigi must drain 90% of the ghosts health in one suction. Draining 50-89% in one go, therefore collecting medium pearls will earn Luigi a silver frame, and 49 and below in one go, with only small pearls will earn him a bronze frame.
The rating system is a bit different for boss ghosts, however. Instead of being based on pearls, it's based on Luigi's health at the end of the battle. If Luigi has 90 HP or higher at the end, he will receive the gold frame. If his health is anywhere in the range of 89-60, he will receive the silver frame. If his health is below 60 he will receive the bronze frame.
Interestingly, 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, The Twins' Room, and Guest Room, respectively.
Fifteen of the Portrait Ghosts have known ages. These ages range from 1-76. There are ghosts of all ages from a few months old to their 70s, except no ghost was in their 60s when they died.