The effect you are describing is called vignetting. It is basically just darkening the pixels with some weight that changes depending on distance from the center of the image. In image editing it corresponds to overlaying the image with black color and applying a circular or elliptic mask to it, for example:
(source: johnhpanos.com)
You can do this by several methods depending on how you operate with image and its pixels. For example by multiplying the pixels by a weight coefficient that is smaller when closer to the center and bigger when farther away from it. The distance can be calculated from the difference between pixel coordinates.