I have a photo that I want to make appear like it is moving behind a static object in a video. I can place a photo on the top most layer of the project, and mask out the object below it with a simple rectangular mask. When I go to move the photo, the mask also moves. Is there a way to "unlink" the mask from the movement of the photo on the top layer?
Don't mask the photo. Duplicate the object layer, and carefully mask around the object on the uppermost layer. Animate the photo between the upper & lower object layers.