You have lots of choices - right in the file node, you have Color Gain (add) and Color Offset (multiply) values to adjust, which can increase/decrease the brightness or contrast or tint the image. You could pipe the output of the file node through other nodes like gammacorrect or clamp (as mentioned above), or pipe the color channels through Ramp nodes (go in as U coordinate for a U ramp) and get lots of control over remapping the transition from bright to dark through controlling that gradient.
-jeremy