On the photo page, on the bottom right click "set as primary photo."
If the photo is saved to a different profile or page, on the middle left of the photo page add the profile ID to the image:
Edit Image Details
What details can you add?
This should be a descriptive headline, e.g. "Jack and Jill on their Hill".
People & Things in the Image:
Add person or free-space profile: ENTER THE PROFILE ID HERE