He is seen wearing light-coloured clothing and a hat.
The death toll from last Tuesday's attacks on the airport and metro rose to 35 after four people died of their injuries in hospital.
More than 300 people were also injured in the attacks, which were claimed by the so-called Islamic State (IS) group.
The death toll does not include three attackers, two of whom blew themselves up at the airport and one in the metro.
Investigators have not commented on reports in the Belgian media that the third airport attacker is Faycal Cheffou, a freelance journalist arrested on Thursday outside the prosecutor's office.
On Saturday a man named Faycal C was charged with "participation in the activities of a terrorist group, terrorist murders and attempted terrorist murders", a prosecutor's statement said.
Separately, three men were charged on Monday with belonging to a terrorist group.
The three, whose names were given as Yassine A, Mohamed B and Aboubaker O, were arrested during raids on 13 addresses on Sunday. A fourth man was released without charge.
Belgian Health Minister Maggie De Block announced the latest deaths in a tweet: "Four patients deceased in hospital. Medical teams did all possible. Total victims: 35. Courage to all the families."
Of the 35 victims, seven have still to be identified, the country's crisis centre said on Monday (in French).
At least 12 of the victims are foreign nationals from the US, the Netherlands, Sweden, Germany, France, the UK, Italy and China, it said earlier.
More arrests have also taken place in relation to what authorities say were planned attacks on France.
A man already in Belgian custody was reported to have been charged in connection with a foiled attack in the Paris region.
Separately, Dutch police announced on Sunday evening that they had detained a Frenchman, 32, in Rotterdam at the request of French authorities.
He was arrested on suspicion of preparing an attack in France and will be extradited to the country. Three other people were also detained.
The Frenchman is allegedly linked to Reda Kriket, who was arrested in a Paris suburb on Thursday and said to be in the "advanced stage" of plotting an attack, AFP news agency reported, citing a police source.
Belgian police have released CCTV footage in a bid to identify the third attacker at Brussels airport, whose bomb did not explode and who fled.