I think B and C are a little more complex than just "one over the other."
Some images in DWM previews and Big Finish covers try to depict scenes from the stories straight, and some try to be more of a "collage" sort of thing. We should value images that were meant to depict the situations over images meant as a "publicity" still.
File:7RaineMasterTARDISes.jpg and File:Dwm_Sword_of_Orion.jpg will always be preferable to images like File:Hex Signs and Wonders.jpg and File:EDA2.jpg.
If anything, I would say that DWM illustrations should be regarded higher than Big Finish cover images because DWM usually represents real scenes and not just cool images of the cast.