I was thinking about a different route - going "artistic". Instead of trying to clean up a poor image to make it look like a good photograph, make it look like a drawing, or etching, or painting, or....
Now I am not a master of this area myself, but maybe someone else can chime in with some other ideas. I have seen some great covers on this site that would be better examples of what I had in mind.
This is what I did with the image below: I cut out Gary, then duplicated it. I applied a Colored Pencil filer to one, then a Cutout filter the other. I set the top layer (Cutout) to 33% opacity, then merged the layers.
I was thinking that you would probably want to do some other artistic filter on the background, because when I did them as one, I did not like the results.