Very simple just a grudge background, give your render some depth by duplicating it Filter>Blur> Gaussian Blur> 20, now you gave your render some depth. Now find where your lighting should hit on the render, and grab a gradient make it FG to Transparent make sure white is your FG, then drag it to where the lighting hits. If you want this on the other side you may give it a shadow just to even it out. Sharpen it up, add a border, some text, and there you go.
Sincerely,
Wenduo