Shamima Begum and Amira Abase, 15, and Kadiza Sultana, 16, flew from here to Istanbul on February 17.
The images show them waiting at Bayrampasa bus station.
Scotland Yard believes the Bethnal Green Academy students from East London are now in Syria.
They are thought to have been met at the border by IS militants.
The girls waited in the offices of two bus companies at the station before taking a bus to Urfa - close to the Syrian border - on February 18, the BBC reported today.
More From This Section
Time codes on the CCTV images suggest the girls were at the bus station, which is on the European side of Istanbul, for nearly 18 hours.
They had flown from Gatwick to Turkey after telling their parents they were going out for the day.
The girls' families have made a number of emotional appeals for them to return home.
Turkey's Deputy Prime Minister Bulent Arinc had said officials would have taken "necessary measures" to intercept the girls had they known about them earlier.
Scotland Yard has denied taking three days to inform officials in Turkey about the girls' planned journey.