May i confirm that all hidden CGs (the feet scene doesn't count cuz it's on twitter) involves clicking "pop-ups" of items, right? I've played around a few times and i think i've found them all.
Yes, in the first chapter you can get most optional scenes by examining the items that appear during specific sections. The feet one you get by examining the scenary and the tickling one you get by choosing a certain option. I made them all really easy to access so the player is unlikely to miss them, but for the next chapter, I'm thinking of making these scenes harder to get to. i.e: making them route specific, or tie them to puzzles. That should give the player a nice sense of achievment/reward.