If you want to get technical, you're both wrong. It goes in order of recent selection. Manually select some tabs in a random order, then ctrl+tab, you'll see it bounce around, starting with the one you most recently selected & then working backwards.
Now, since newly opened images open in tabs to the right this means that if you open a bunch of images at once, ctrl+tab will go from right to left. (At least unless/until you manually select some in a different order.)
Oh the feuhrer, oh the feuhrer, oh the feuhrer's nipples bonk!