I have the 128GB Retina Mini AT&T version and it does not have any image retention. Don't go to a site that makes you view an image for ten minutes either. The true test is if you have image retention from just simply using the iPad.
I frequently leave my Mini on with one app open while doing other things, even using my Air and then come back to the Mini. No retention at all.
That's not really a reliable test since you have to keep that image on screen for ten minutes. No one in real life would ever view that same image for ten minutes in anything you normally do.