How do I fix image persistence

Have a look at > Avoiding image persistence on Apple LCD displays

Image persistance on Apple displays:

http://support.apple.com/kb/HT5455

Apple say this about LCD screen burn in:

http://support.apple.com/kb/HT2807

which includes suggestions for curing that effect.

Alternatively:

How to fix screen burn and/or stuck pixels on an LCD monitor:

JScreenFix

http://www.jscreenfix.com/basic.php

Hey!

I've had the same problem (image persistence) on my iMac 27'' (late 2013).

I had to have my display 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!