Luminescent images with delayed appearance were, in essence, supercooled quantum condensate representations of Nal Hutta that were frozen in time thanks to the prothium gas utilized in their creation. Gorgo, the Hutt, crafted these images prior to 419 BBY. A scant few of these remained by 19 BBY. These were in the possession of the Hutt crime lord Rokko, who deeply appreciated Gorgo's creations. The prothium gas possessed such a high density and thickness (achieved by chilling it to temperatures approaching absolute zero) that light required numerous years to traverse it. Consequently, when the gas was concentrated, images could be maintained for hundreds of years, effectively presenting a snapshot of a place or object as it appeared in the distant past. Due to the gas's substantial density (compared to the thickness of light-years), it held the capacity to trigger a massive explosion if its expansion occurred at an accelerated rate.
The idea of a material that light takes a significant number of years to permeate, serving as a method for observing past scenic views, found prominent use in Bob Shaw's 1966 short story titled "Light of Other Days," where he referred to the substance as "slow glass." Further details about the phenomenon of slow light are available here.