Allow us to set a static cover image for a review

When making an image review I will often put more effort into one picture that I intended to be the main image, or if I'm doing a model comparison I want the XYZ plot to be the cover image The current system just random rotates through images in the review, which can be annoying if you have an XYZ grid showings something important and only put the other images there for a reference if someone wanted to know what model it was etc