Portrait flip your phone in full screen mode. 

1- To stop the slideshow; Move the mouse over the picture , use the arrows or swipe.

2- To resume the slideshow, on mobile; double tap. On desktop; move the mouse away or Exit the gallery.

3- To enter the gallery; Click on a picture. For Full screen; click the icon a the top  . To exit the gallery; Click the X at the top .

4-While inside the gallery;  Use the arrows (fast) or swipe while in full screen option [ ] . There is a zoom tool option, or pinch zoom on mobile. On desktop you can hold ctrl and mouse-wheel zoom.  Use the address bar zoom% to reset.  

5- To restart; Refresh/ reload your browser.

The polar bear picture is at midway .