What is Dark and light imagery?

Dark imagery is the use of darker, and colder Words as opposed to brighter more friendly ones. For example Dark imagery could be like 'The castle had a frozen feel to it, within the corridors lurked the most vicious of creatures, as if to reflect the owner of the castle, this place is more of a cage to me than anything, the my fingers tremble in the merciless cold, No one is coming, no one ever was, and until then I will tremble, pathetic, dirty, repulsive, creatures that roam the atrociously ugly creatures dying from the inside out, creatures that cannot ever blend in, join the group, always to stand out and be forever looked over, creatures like myself' the ending provides a terrible and hopeless image to forever bathe in the darkness and brings darker and lonelier feelings into the mind of the reader.