How to turn off an image in WPF?

hi fellas, I mean, how to show it in grey?

<Image Margin="435,3,150,267" MouseMove="Image_MouseMove" Source="/Imagenes/onoff.png" Stretch="Fill" Grid.Column="4"/>

I want to show this image turned off...

thanks for  your comments

Primary platform is Windows 7 Ultimate 64 bit along with VS 2012/Sql2k8 for WPF and SilverLight stuff