Called tereg in the Sindarin language of the elves, the trolls of Middle Earth were a giant, cannibalistic race of strong, fierce, but dull witted individuals. Their hides were made of overlapping scales which could resist all but the sturdiest of weaponry. In the same way that Orcs were bred as a mockery of the world of Elves, Trolls were bred to parody Ents and used for evil by Melkor. They were known to hoard treasure, kill for entertainment and eat any form of raw meat they could get their hands on. Their origins lie in rock and stone, and it is to this state that they revert if exposed to daylight, not being used to sunshine as a result of being created in the darkness of the First Age. Many breeds of troll were used in the War of the Ring, and a Cave troll would have killed Frodo in Moria if not for the protection of his Mithril shirt.
The trolls depicted in the painting are the stone trolls Bert, Tom and William Huggins, first encountered by Bilbo and the dwarves in The Hobbit. At this point they were roasting mutton, and it was the light from their fire that drew Bilbo towards them. They captured him practicing his thieving skills by trying to steal William's wallet, and are preparing to cook him when a fight breaks out between them. They pause their fight only when they see Balin and proceed to trap all the dwarves in sacks. They then resume their argument, this time with the topic being the best way to cook the dwarves - to roast, mince, boil, or squash them to jelly. When they have finally settled on the idea of roasting, Gandalf fakes the voices of each of the trolls to create further argument, and the trolls are so distracted that they do not notice the sun rising and are turned to stone.
The painting shows the trolls as Frodo, Sam, Merry and Pippin find them, frozen in their eternal state of dispute outside their home.