I have made a guide for all 20 hidden idols and that is under its own achievement but here is the list.
You obviously only need 5 rather than all 20 of these but you might as well get all of them for two achievements!
The hidden idols unlock concept art and you will find one of them in each level. They are not particularly hard to find and are glowing red. Here is a description of locations if there is one that you are missing.
1. A New Hop - First forest area, bounce on the mushroom on the right-hand side.
2. A Succumbing To Age - After the statue telling you about backflips, kill the Spybot and drop down for the idol.
3. A Zero's Journey - When approaching the Temple of Doom, there are some crates with the idol hidden inside.
4. The Temple of Zoom - Before the boss fight, you have to cross a large gap. Just hover to the idol.
5. The Emerald Pity - When sliding down the first big hill, stay to the right to find the idol.
6. Don't Eat The Yellow Brick Load - When you cross the gunk-filled gap using the logs, the idol is on the left-hand side.
7. The Blizzard Of Claws - In the final boss fight on the left-hand side near a breakable fence.
8. Life's A Beach - Straight after the cutscene, you will see the idol in front of you.
9. Operation Overboard - After fighting the Scoropilla and before the mojo room, you slide down a passage. At the fork in the tunnel after it, go to the right.
10. A Sludge Too Far - Climb the wall after the orb and then move across the vines. When you get to the end, double jump off over the pink plant for the idol.
11. Shock And Awesome - Make your way through the rooms in NGin's office.When you get to one that you have to swing to the left as you come in, you will see the idol to the right.
12. Weapons Of Mass Construction - Simply double jump at the start of the level for this one.
13. Doomraker - Shoot the generator in the bottom right corner of the first battle are to find this idol.
14. Family Tree - When jumping across the tilting platforms near the start of the level you will see the idol.
15. Calamityville Horror - After the mojo room, you can bounce on a mushroom for this one.
16. Timber Trials - About halfway down the chute at the start of the level, jump and grab the idol.
17. Adolt Edumacation - After the mojo room and Ee-lectric, you will find the hidden idol behind a node.
18. War Of The Whirls - Right at the end of the level when you kill the Spybot, you will see the idol.
19. Minority Rapport - Between the fourth and fifth armies, you slide down a chute with lava-filled gaps. The idol is over one of these.
20. Revengeance Two: The Revengicide - In the first arena after the mojo room. Kill all the enemies and then double jump from your titan when standing under the red object.
I hope that these locations are understandable for those following it, PM for any help. Good luck.