I do not mean modern art which does something like that humorously but rather for example a medieval painter showing Pharoah's army equipped with muzzleloaders due to the artist being unaware of the time when such weapons were first invented.
In history "ancient" usually refers to the time period of Antiquity, the time before the beginning of the Middle Ages roughly about AD 500.
I note that King Arthur was said to have been killed at the Battle of Camlann, which is date to about AD 537/538/539 by the Annales Cambriae and to AD 542 by Geoffrey of Monmouth.
And some definitions of the start of the Middle Ages put it after AD 600, which would make King Arthur one of the "ancients".
In Le Morte d'Arthur of c. 1470, there is a description of a siege, probably Mordred besieging Guenivere in the Tower of London, mentioning "gonnes" or guns, cannons.
Thus I can imagine that possibly some illustrated editions of Le Morte d'Arthur included images of battles and sieges with cannons.