Image selection begins with purpose. An image for an introductory page needs a clear focal point; an image for a detailed study can reward longer looking. Define that job before comparing alternatives.
Edit in more than one pass
First remove frames with distracting technical problems or accidental expressions. Next compare the remaining images for meaning, gesture and fit with the surrounding material. Finally inspect the chosen photograph at its actual display size and crop.
Keep the strongest alternative until the layout is settled. A slightly quieter frame may work better beside text or within a sequence than an image that dominates everything around it.