Novel by Graham Masterton (Scotland) 1985.
Art gallery owner Vincent Pearson owns a portrait depicting twelve members of a family who all seem to be decaying due to the artist's poor technique. It turns out that these twelve comprise a family known as the Grays, who took part in a black magic ritual which rendered them effectively immortal by enabling their painted counterparts to age in their place. Unfortunately as the painting is decaying badly, the real life Grays only appear young whilst really rotting away from the inside out. Thus they periodically have to kill people and deprive them of their skin to wear over their own rotting flesh.
Superbly written in his usual style by Graham Masterton, who once again proves that, with the possible exception of Brian Lumley, he is the most criminally underrated horror novelist in existence. Could it be a coincidence that both authors defy categorisation: Lumley writes sf, heroic fantasy and horror, frequently in the same novel, and Masterton is a successful and best-selling thriller and family saga novelist. This book, however, is most certainly genre material: his horror is horrific, the maggot scene is cringe-inducing, and the rest of the plot- and character-building is admirable. And before you cry plagiarism, the similarities to Oscar Wilde's The Picture of Dorian Gray are deliberate and fiendishly worked into the plot.
Published in the USA as Picture of Evil.