How to make an image darker or lighter (useful when creating icons)

Sometimes we want to make the image darker or lighter to create mouse over image or diabled image.

Here is how we can do it using GIMP:

You can get to this tools in several ways:

• In the image menu through Tools → Color Tools → Levels.
  
• In the image menu through Colors → Levels.
  
• By clicking on the tool icon in the toolbox if this tool has been installed there. For this, go to Tools Dialog.
  

Detail: http://docs.gimp.org/en/gimp-tool-levels.html