Doomender: Firstly, try saving your signatures as the .PNG file type, it's the highest quality. Simple lighting is easy, just choose a 300 pixel soft brush and use it once on a different layer on top of the render, or wherever you want your light to be coming from.
Here are a few of my examples:

A difficult render to use because of the lighting on Voldemort's face/body, so I added a lightsource in there so it would be blended a bit better.

See? Very simple effect.
gin007654321: Do you have a version of Photoshop?