image persistence on imac 27"

it's called "ghosting" the following article tells how to correct this:

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