The flag in the picture was of what country or empire?

The Benin Empire"1180-1897" was a pre-colonial empire located in what is now southern Nigeria. Its capital was Edo, now known as, Benin city, Edo. It should not be confused with the modern-day country called Benin.The Benin Empire was "one of the oldest and most highly developed states in the coastal hinterland of West Africa, dating perhaps to the eleventh century CE", until it was annexed by the British Empire in 1897.

The putative Flag of the Benin Empire is a West African flag that was allegedly brought to Britain by Admiral F. W. Kennedy after the 1897 Benin expedition. The original flag is currently held by the National Maritime Museum in Greenwich near London. Little is known about the flag.

The Greenwich Museum, to which the flag currently belongs, describes this flag as follow:

West African flag probably Itsekri. A red wool bunting flag with a linen hoist, machine sewn with a rope halyard attached. The design is applied in white fabric with painted details, representing a naked man decapitating another with a sword. The flag was said to have been brought back by Admiral F. W. Kennedy from the 1897 Benin expedition. 'Kennedy' is inscribed on a paper label attached to the rope. The Itsekri people acted as middle men between the Edo people of Benin in the interior and the Europeans on the coast – the Edo would not cross or travel on waterways