I couldn't find a good stand-alone full image of this item. A cropped version is often used to illustrate the use of awnings in amphitheaters, so that part may be familiar.

The painting shows the story of an actual event, the Nucerian Riots of 59 AD ( alternate image (cropped, but better color)).

Sparked by a gladiatorial games rivalry, the disorders led to the sport being banned in Pompeii for ten years (rescinded in 62 AD as a relief measure after an earthquake).

The painting shows a panoramic view of the riots -- the gladiators fighting in the arena, the fans fighting in the stands, other citizens fighting in the streets, and the aftermath: an inset at the bottom of the magistrates handing down their ruling.

Visit the SPQR Companion for all things Roman (including spoiler links).