How can i blur an image?

Hello, i'm trying to apply a gaussian blur effect to an image (.jpg), like this example: notes.ericwillis.com/2009/10/blur-an-image-with-csharp/
But it throw me: Error CS0246: The type or namespace name `Bitmap' could not be found. Are you missing an assembly reference? (CS0246)

I would really appreciate any help.