Certainly you can layer still images and even video, Jonathon. But you'll need to be more specific or show us an example of the effect you're trying to create to get specific instructions.
Are the objects that will appear behind someone video or just graphics or still images that you will animate?
What I understand from your question, most certainly key framing motion and scale property for image/video will work for you. But to answer it more clearly and to guide you in a refined way, we would need a little more information.
Welcome to the forum.
For the use of another backgroun (set of Stills, if I understand correctly), you will also want to Key out the original background, around the runner, UNLESS you just want to add the "objects" TO that background.
In the latter case, you will basically be doing PiP (Picture in Picture), where those objects will be on Video Tracks, above the background, and then they can be animated with Keyframes on the Fixed Effect>Motion>Position, and probably Motion>Scale.
I agree that an example of your desired treatment would be useful in giving detailed instructions.