1) On 1 layer, I have the photos with a Liquid 2D CPU Clip effect on each photo (in order to resize, add border, shadow, etc....)
2) On the layer under the photos, I have a jump back background animation. To this point everything looks fine.
3) My next task was to add Boris Red transitions between the photos. When I create the transition in Boris, I make sure that the background layer is transparent (I see gray and white checkerboards...) When I render the transition, the background is black, therefore covering up the jump back.