IMO a much larger rectangular picture would look better & more in balance for that space.

I think the mantle itself is odd and takes away from the look. Replace it with a metal mantel and some appropriate fireplace accessories and some type of artwork above, be it metal or painted. I would have suggested a wood beam mantel, but you cannot use anything combustible that close to the opening, which explains the odd concrete shelf.

Custom Fireplace Accessories by: Fireplace Treatments

I think your dog is the perfect decor.

Something with a traditional layout. Here is a bolder version.

Is it a low ceiling or an optical illusion?