Some camera lenses produce photographs that are darker at the edges than in the middle.
Here's how to fix that in Photoshop by using an adjustment layer. Open your photo in Photoshop and choose New Adjustment Layer from the Layer palette's pop-up menu. Set the foreground color to black and the background color to white. Select the radial gradient tool and drag the radius from the center of your photo to one of its corners. Adjust the sliders to reduce the darkness at the edges of the photo.