The photo shows Masood with a short beard and moustache and a bald head.
The only pictures of the 52-year-old until now had been of him on a stretcher being attended to by paramedics after he fatally stabbed PC Keith Palmer in the grounds of Parliament.
Masood's birthname was Adrian Russel Ajao and he also went by the name Adrian Elms and a number of other aliases.
On Wednesday he mowed down dozens of pedestrians on the west side of Westminster Bridge before crashing his rented Hyundai 4×4 into railings surrounding the Houses of Parliament.
We named the dead terrorist as Khalid Masood, and as I said, our investigation focuses on understanding his motivation, preparation and his associates
Acting Deputy Commissioner Mark Rowley
He then rampaged through the open gates into New Palace Yard, fatally stabbing PC Keith Palmer with two knives.
Two of the pedestrians died on the bridge and another died in hospital yesterday.
Acting Deputy Commissioner Mark Rowley has appealed for the public’s help in the investigation.
He said: “We named the dead terrorist as Khalid Masood, and as I said, our investigation focuses on understanding his motivation, preparation and his associates.
Masood killed four people during his rampage in Westminster
"Whilst there is no evidence of further threats you will understand our determination to find out if he either: Acted totally alone inspired by terrorist propaganda; or, if others have encouraged, supported or directed him.
"At this point I want to appeal specifically to the public.
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"We remain keen to hear from anyone who knew Khalid Masood well; understands who his associates were; and can provide us with information about places he has recently visited
London terror attack: Latest pictures Fri, March 24, 2017
4 dead and 40 injured in London terror attack
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Armoured police personnel carriers are seen on a street leading to the Houses of Parliament in central London on March 24, 2017 two days after the March 22 terror attack on the British parliament and Westminster Bridge
"There might well be people out there who did have concerns about Masood but weren't sure or didn't feel comfortable for whatever reason in passing information to us.
"I urge anyone with such information to contact us the Anti-Terrorist hotline which is 0800 789 321.”
Detectives are continuing to search two addresses in Birmingham and one address in east London linked to the investigation:
Scotland Yard said other searches at addresses in Brighton, south east London, east London, Surrey, Manchester, Camarthenshire and Birmingham have concluded.
Acting Deputy Commissioner Mark Rowley has appealed for the public's help
Floral tributes have been left to Masood's victims in Parliament Square
11 people have been arrested as part of the investigation.
A 39-year-old woman was arrested at an address in east London on suspicion of preparation of terrorist acts. She has since been released on bail until a date in late March.
A 21-year-old woman and a 23-year-old man were arrested at an address in Birmingham.
A 26-year-old woman and three men aged 28, 27 and 26 years old, were arrested at a separate address in Birmingham.
All six were arrested on suspicion of preparation of terrorist acts.
Police said the arrests were made overnight on Wednesday.
A 58-year-old man was also arrested on the morning of Thursday, 23 March at a separate address in Birmingham on suspicion of preparation of terrorist acts.
A 27-year-old man was arrested late on Thursday at an address in Birmingham and a 35-year-old man was arrested in the early hours of Friday at an address in Manchester.
A 32-year-old woman was arrested this morning at a location in Manchester. All three were arrested on suspicion of preparation of terrorist acts
Ten people remain in police custody. Nine were detained under the Terrorism Act and one was detained under the Police and Criminal Evidence Act.