Help in making rollover Image stay

hey guys, i need some help. i need to make a rollover image stay when on click using javascript, any ideas on how to do that?
 
For example, when i click on the AboutUs Image, it will become darker a bit, when i click on another page, the AboutUs will be the normal colour back.
I'm using masterpage and C# to build this website. Any Help? 
 
Thanks so much..
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