1. Categories should be allowed to add a Grid Card Image.

2. If a category has a Grid card image then it should suppress the images for the child forums card.

Each forum, sub-forum get's it's own grid image. If there's sub-forums under a grid category it should show just the sub-forum category name text. That's normal. If that's not what's happening to you then I'm not sure.

The root Category has no means to add Grid Card images. 

• Category (not allowed)
  • Category (allowable)
    • Forum (allowable)

If you have tons of forums grid view is a bit much but if you have a handful of root categories - they could be a grid and outline contents with image and perhaps.