I have Nov 2012 Imac which has started showing strange screen artefacts. If I leave an object on the screen such as Safari without moving it for a while (say 20 mins), it leaves a greenish ghost image after I close or move the application on the screen. This can take a while to disappear. Has anyone else noticed this? Is this something that I should be worried about, or can it be stopped by simply changing a screen setting somewhere? Any help is appreciated.