Maybe it is the colour of background you have set for your Renderosity settings.That can change the contrast from what your eyes see.I have mine on black always.So possibly it is not your image,but the suroundings. Also if you have a LCD monitor the difference in height you view changes the lightness. These are just my guesses.

"The happiness of a man in this life does not consist in the absence but in the mastery of his passions."