Many displays in imacs have image persistence problems this is a very common problem. Typically the panel works fine for 6 months to a year and then they'll start leaking. Either DONT BUY an iMac or buy extended apple care, otherwise if this happens to your panel (and its pretty common) then you're screwed. This happens in all iMac models. Retina, non-retina, seems that LG makes terrible panels. It might not happen to EVERY iMac sold obviously, but it's a very common problem with iMacs.