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.