Decelerated luminescent images were essentially supercooled quantum condensate pictures of Nal Hutta frozen in time due to the prothium gas used to make the images. These images were made by Gorgo the Hutt, before 419 BBY. By 19 BBY, only a few existed. They were owned by the Hutt crime lord Rokko, a great admirer of Gorgo's works. The prothium gas was so thick and dense (by cooling it to near absolute zero) that it took years for light to pass through it, and when the gas was concentrated, images could be preserved for centuries, so that the image would be a picture of what that place or object had looked like years ago. Because of the density of the gas (likened to being light-years thick), it has the potential to create an enormous explosion if expanded too rapidly.
Behind the scenes[]
The concept of a substance through which light takes many years to pass, used as a means of viewing scenic vistas from the past, was famously used in Bob Shaw's short story "Light of Other Days" in 1966; he called the substance "slow glass." More information on the slow light phenomenon can be found here.