SUMMARY In fill mode only up/down or left/right is used at the same time so the other two could be used to go to next/previous image. In 'Zoom' mode all four arrows are used to move the image so I would expect to be able to use them with Shift or something else. Version 19.11.90
One can still use Space and Backspace to go to previous and next image. I didn't notice this at first, because it's not very intuitive.