Morkredd

In addition to the other solution, I've made a clip that shows where you can find the mural in the level "The Meat".



Note that there isn't a Chapter Select option when you finish the game, so if you miss the mural you have to replay the game.

Around 85% into the game, you'll pass through two small light-sucking villi patches into another area with a large patch and a lot of the light regenerating blisters (?). Hug the north wall of this area, taking advantage of the blisters and the patches of villi-less floor to let your light regenerate when you can. Up a small walkway a small way round the wall you’ll find the secret mural in this area.