You have selected the fullscreen mode to view photo. If you like to view in this way, you can set option 'Always open in fullscreen' in 'Tools-Settings' menu item.
Under fullscreen mode, you can switch to next/prev photo by press '->','<-' or 'Pgup','Pgdown'. Using the wheel button on mouse can zoom in/out the photo. Press 'ESC' to back to normal status.