I would blend the teeth of goku in so the image in front doesn't have a transparent look. I would add white smoke around goku to help blend him in with the background. If you are using gimp click on render and then Lava. A new layer will be added, Hit the colorize button to bring it to the same color as you background.Next drop the opacity of that layer and add it as either gain merge or addition. Erase the lightning around the sig as needed to help him blend in.