how to fix position of bounding box of an image

I want to superimpose a photo of myself lying down on the pavement (where I fell), on another photo showing the place where I fell. It is important that the position of the first photo does not change along the timeline. My focus in this query is on the first photo.

I have fixed the Transform position at two different points in the timeline. The numbers do not change, but the bounding box shifts its position and I cannot figure out why or if there is some other parameter that needs adjustment. I attach a collage of two snapshots showing how the bounding box changes without there being a change in parameters.

Your help will be appreciated.

Francisco