No, no cannons then. The painter is painting what he knows from his own time. The painting is done in 1537 and although of an earlier siege in 626 it looks more like the fall of Constantinople in 1453. For the record cannons are attested in records in Europe around the 1320s.

What you observe is an artistic convention very common in medieval times. Artists would dress historical figures in modern (to the artist) clothes as well as basing other details on their contemporary knowledge and environment rather than what they did or did not know of earlier historial periods. With healthy dose of stylistic imagery we don't always understand or pick up on.

Which is why you can get pictures of Roman legionnaires fighting Greek phalanxes that look like something out of the 100 Year War. And this is despite the creators being completely able to know what Romans actually looked like due to extant Roman remains lieik Trajans column..

One of the more famous examples is the Morgan or Maciejowski Bible where biblical events are depicted as if they had occurred in the mid 13th century when the illustrations were made.

This of course makes them useless for information about the past they claim to depict but provide us with sometimes astonishing details of contemporary (to the creation) weapons and armour and such. Military historians and people creating miniature wargames pour over these old documents and wage their own wars of interpretation (every bit as savage as the wars depicted) as to what exactly that weird curved thing there on folio 10r is. E.g. the bible I mentioned earlier has an illustration of a helmet being cleaved and I swear that question is not going to be settled with less than one academic side cleaving the literal helmet of the other.