i think linux doesnt save the image in the clipboard after you pushed the button, in ubuntu however a dialog is displayed after pushing the button that allow you to copy the screenshot to the clipboard, but this tool is specially designed for this purpose. AFAIK this wont work out of the box in linux :(
Well I guess that's expected behavior. So you may need to explicitly state that the feature works if you save an image on your clipboard -- not just pressing printscreen ;-)