"Oooh, very good, it took Diggory much longer than that! And that was with her awake too, giggling and showing off and flashing her fins ..."
-- Moaning Myrtle (GF25)
A gold-framed painting of a blonde-haired mermaid hangs in the Prefects’ Bathroom on the fifth floor of Hogwarts. She sits on a rock and is usually asleep and snoring. She flirts with the young gentlemen while they bathe, which doesn’t seem to impress Myrtle much (GF25). She resembles the mermaids of folklore (that is, like merfolk who live in warm climates such as the Mediterranean Sea (FB)) rather than like the Selkies living in Hogwarts’ own lake (GF26).
The mermaid in the painting appeared in Harry's fevered dream some hours later after he fell asleep researching how to breathe underwater. In the dream she grabs his Firebolt and won't let him have it back, instead poking him with it (GF26).
The mermaid on the rock is reminiscent of the famous Little Mermaid statue in Copenhagen harbor.