I've had the same problem (image persistence) on my iMac 27'' (late 2013).
I had to have my panel changed through Apple Care a few days ago. The thing took 30 minutes.
Apparently they solved the problem on later models.
If your iMac is still guaranteed I would strongly suggest that you have it serviced (whether you have Apple Care or not). It is not a software problem that you can solve using the white screensaver!
It is a hardware problem.
Hope this helps!