"Portrait of a Dead Witch" is a painting by John Yeadon. It shows WITCH, previously known as the Harwell Dekatron computer, which currently holds the record as the world's oldest functioning digital computer -- the witch having "got better", as they say.
The computer itself is in The National Museum of Computing at Bletchley Park, while the painting is in private hands, hung on the wall of the Jam Street Café in Manchester (209 Upper Chorlton Road, if you'd like a look).