There're 7 unmissable murals, and 5 "secret" murals throughout the game, with each secret mural having its own associated achievement. This achievement will pop after coming across the last "unmissable" mural in the final level (around 89% in, thanks xNeo21x). There are guides for each secret mural within each individual mural's achievement guide but I'll also post each one below within its own spoiler tag so that they are all contained within this one solution.
1. The Sewer
*** Spoiler - click to reveal ***Near the end of the level, you'll cross two pipes where the last of the worms spawn. A small cutscene now kicks in where a mysterious figure pulls a lever to drop part of the floor. Move past this and to the end of the walkway where you’ll find a small bridge. Cross this with one character and move back up the way you came toward the lever, moving up when the pendulums are raised to their highest points, all the while the other character moves the orb back up the other side to keep you in the light. Once you get to the lever, pull it, and bring the floor back up to cross the orb over. Instead of continuing down the natural path to the left here, take the orb back down the path to the right closest to the camera, past the pendulums and the bridge, to come to your first secret mural in the game.
2. The River
*** Spoiler - click to reveal ***Around 32% into the game, after the double-lock puzzle near the start of the level, you'll exit the lock into a "rapids" section where the water is flowing quite fast. Hug the wall at the top of the screen and continue round until you find a waterfall. Proceed through the waterfall to see the mural.
3. The Mountain
*** Spoiler - click to reveal ***At around 49% into the game, you'll come to a section just after a lift with a bunch of extremely long freaky legs resembling a forest of Peter Crouches, where you can pick up the "Tickling Feet" achievement. Towards the top of this area, there is a slightly hidden path with a foot in the entranceway. The path leads to another of those horrible feet, but also, more importantly, to a cave, in which you can find this mural patiently awaiting your arrival.
4. The Temple
*** Spoiler - click to reveal ***Around 62% of the way into the game, following a section where you follow a slug through a set of three windchimes, then another set of longer windchimes, you reach an area where you must pick up an artifact to continue, as the slug you were following veers off to the right into a hole.
After collecting the artifact, continue ahead into a rotating circular room that we’ve seen before in the Sewers, so rotate the opening to the right to allow you to proceed. Drop the artifact on the pedestal here to open the door.
In this next section, one character needs to head up the stairs to the left and hit the switch to open the door. The other character heads through the open door and stands on the next switch here to complete the wall (careful with lamp shadows here), but not before viewing the mural on the back wall here. The top character can now run along to the third switch to open the next door and allow the bottom character to safely pass through the now open doorway and move towards the edge of the light.
Ahead you will notice a crystal lying on the ground, so collect it with one of your characters. Now work your way back to the rotating circular wall mechanism. There are a set of stairs off to the back of this revolving room which you can rotate the opening to. Take the character with the now-acquired crystal down these stairs and loop back on yourself to the left. Down this hallway is the Temple secret mural, painted on the side of the set of stairs you've just descended.
5. The Meat
*** Spoiler - click to reveal ***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.