I have placed a number of stills on the timeline on Video 1. Above some of these stills I have placed other stills on Video 2 which use alpha channel to become transparent and therefore see part of the still below. If I try to add a still halfway through the timeline it moves all the remaining stills that relate to each other on video 1 and 2 out of synch.

Is there a way to lock stills that relate to each other on video 1 and 2 so I can add further single stills to video 1?
Thanks - Julie