Hmm, cant be a good make of monitor if it leaves ghost images for that long. Something seems wrong there
Are there any digital settings within the monitor accessed by buttons that might help?
I can't really say anything to help except to look into the settings on the monitor though, as it won't be a problem with your PC, etc.
If it keeps doing it after looking around the settings, it might be a fault, since some pixels aren't switching off.