The photo navigation bar, enabling movement between photos within a gallery, sometimes disappears fom the bottom of the window.
It will normally reappear if you click anywhere on the bottom of the window.
You can also navigate between photos by:
On a touch screen, when in a photo gallery, you can move between photos by "swiping" left or right
On a non-touch screen you can do the same by using the cursor/mouse for "swiping", holding down the left or right mouse button as you swipe.