Thanks Michael,
I took some expression class some time ago. I’ve forgotten most of it but I do remember it was possible to write an expression that if one object intersect with another its opacity will change. Maybe that would be a way, using a big solid for example as an “effector” effecting the opacity of photos to reveal them (by moving the solid) … The photos would have that expression.
Though, your solution seems to be much easier to execute. But how would you make sure that banding aligns with photos? My photos are sort of rotated randomly and overlapping each other a bit (so it more looks like a random pile).