Activate full screen by clicking the rightmost button below the picture.
Press and hold the left mouse button to pan.
Double left click to zoom in.
Use the mouse scroll wheel to zoom in and out.
Use "a" to zoom in.
Use "z" to zoom out.
Use the arrow keys to move around once you have zoomed in.
Use ESC (Escape) to reset to the default zoom level.
(You may need to click once on the picture to activate the keyboard navigation.)