Trevor Joyce, 37, stabbed Mr Skrebowski in a random attack in an Abingdon Poundland on December 7 last year. The Old Bailey was told that Joyce is a schizophrenic and a drug addict.
Joyce was charged with murder but pleaded guilty to manslaughter on the grounds of diminished responsibility.
Joyce has been sentenced to at least nine years before applying for parole. He will stay at Broadmoor Hospital until he is deemed fit to be transferred to prison.
Mr Skrebowski, 61, was a specialist in topographical, natural history and architectural prints as well as antique frames, print stands and easels.
He had been shopping for his wife’s birthday when the attack took place. He lived with his wife and twin children in Oxfordshire.