We went through a lot of this when I first started posting pix using a Mac about three years ago. Some folks thought my pix looked dark. I think it was Jerry Day who pointed out that Macs have a screen that is designed to provide a bit more contrast than PC screens. So after Photoshopping the pix on my Mac I would check them on my PC screen and usually lightening them up a bit. Both happen to be LCD screens. Later on my PC got replaced and the new PC screen seems to have the same contrast and brightness as my Mac.
Interestingly, while redoing my ng. website some of the original files stored on CD's started looking dark to me. Hmm....could CD files start degrading in three years? Or maybe my now three year old Mac screen is perhaps loosing some of its brightness.
Or perhaps my own preference has changed.
Net net, when you add in personal preference, this internet image brightness issue seems to be as much art as science.
JBW