You can insert a picture into the comment (Select comment, right click and then Format comment/Colors and Lines/Color/Fill Effects/Picture). To control the level in a tank, though, is possible only using VBA and a few different pictures.
The other way is to use conditional formatting on a vertical range of cells representing the tank. Let say tank is 10 cells high, the value in cell "B2" represents percentage of tank filled.
The bottom cell condition ="$B$2>0" background color = blue
Sectond cell condition =$B$2>=15
......
Top (10) cell condition =$B$2>=95
See attached file.
hope it helps!