The Picturebox you're referring too is used only as a receiver for the Window Messages sent by/to the Tray Icon,
so doing any kind of graphical operation on the Picturebox will do you no good.

Instead you will have to either update the Icon each time,
or use the "ToolTip" of the Tray Icon (although this way the new value wouldn't be visible unless the mouse is moved over the icon).