GDI+ bitmap opacity

I'm making a small game as a test with GDI+ (C# 2.0). I had this question: is there an easy way to draw an Image object to Graphics with different opacity? I don't mean the transparency key thing, but the whole image should be half visible, and trasparency color of course not visible at all. I managed to alter the opacity of the image with getpixel and setpixel, but I'd like to know if there was a way to avoid pixel manipulation (since it takes time).