A sand picture consists of two panes of glass glued together creating an air tight seal.

 

Liquid, sand, and small pockets of air live inside the glass.  Rotating the sand picture will cause the air bubbles inside the glass to rise.

 

Creating an air barrier between the liquid & sand.  The sand will eventually start to squeeze itself through the air bubbles.

 

 

More air inside the glass = slower avalanche of sand.  

Less air inside the glass = faster avalanche of sand.