You give the page dynamics and direction. Let the shape of the visuals help you decide placement. If your lead photo is a vertical, you may want it to go at the top of the page. If it's a horizontal, it might be better to place it below a story that wraps across the top. In both cases, what you're trying to do is to place the key visual over what is called optical center of the page.
Optical center is a bit above and left of true center.
One guideline is that on an open page the lead art must be no smaller than 24 column inches. Secondary art on the page should be about half the size of the lead art. If there are too many stories, move one or two of them to an inside page. If the stories are too long, segment them or trim them to fit. In an instant.