Changing the saturation of an image
 

There are times when I'm wireframing that I grab an image online, copy it to the clipboard, then paste it in my canvas.

Is there a way that I can change the image's saturation? Meaning remove the color from it? I see that I can change the image transparency, but I need to make it a greyscale image, without round-tripping through photoshop...

any ideas??

Not that I know of... Omnigraffle is primarliy a layout/ wireframing tool, not for image editing. For that you have to do a round-trip to another program.