Certainly not random.
As per the stats page, I've over 10 photos rated with an average of 5*, but they are listed in the same way each time I refresh the page.
Though I accept that this may be down to the caching, the photo that has been "top rated" and therefore shown as the main image on the zpMinimal entry page has remained the same every since I moved to that theme.
Additionally, with the same data on my LAMP system at home, the same image is shown on the front page also.