Images are too dark or too light ?
  "Monitors are not all universally adjusted to the same standard. You know, for example, that there is a "brightness control" and a "contrast control" on your TV and on your monitor. The quality of the image you see is significantly influenced by how these two controls are adjusted on your monitor."  
Calibrate brightness and contrast
On the the chart below you should be able to see the 30 separate tones going from an all white to a total black and no colors should be visible.


or 10 boxes with this one:
Calibrate gamma
On the the chart below one of these numbered boxes is about the same shade as the surrounding.That's your "gamma screen".Pictures on this website are designed for a best view with a 2.0 Gamma monitor.

Test picture
 The picture 2 must now be clean on your screen (n°2)
1 (too dark)
2
3 (too light)


references :Lenswork / Stanstudio / South Bay Camera Club / Adobe