I'd say by chapter 3 add in way to track what you have or have not found, like a CG gallery, but in event its a scene with no CG have a representative (simple) placeholder image (such as the character(s) it pertains to). Then by chapter 4 or 5 add in hints, which may also require enough progression before they show up in this gallery. In any case being at least a little cryptic helps to not spoil the content it hides.
Ah.. the humor never gets old, this is what I love! The awkwardness! XP
And with that.. its delightfully horrible... or was it horribly delightful? What was the point? It WAS the point.