It looks like 2.56 has introduced what was, to be fair, probably the original intention of the multiple image slots. So rather than toggling between two images with <J>, it toggles between up to 8 images. Always one more than you have stored in slots.
For me, I much prefer to have just 2 slots so that I can quickly compare two images. One before and one after a change, usually. Does anybody know whether there is a way to revert to having just 2 slots in use?
To clarify (or probably just confuse), this is what happens now:
I render an image and it goes to slot 1…
… then I hit J and it shows me a blank image screen (slot 2)
… I render again and it goes to slot 2
… then if I hit J I see another blank image screen (slot 3) and I have to press J one more time to get back to my original render (slot 1)
… so I render again and this time it goes to slot 3, and pressing J brings up yet another blank image screen (slot 4)
… and so on.
Thanks.