I find the easiest way to do graphics is to create PNG files. I personally used Adobe Illustrator to draw them, but it does have a steep learning curve. I have seen others use Solidworks.
In the latest beta version there is a new control called the Graphic Bar Level. It lets you draw a graphic that is filled instead of the standard rectangle bar level. It lets you do things like give the effect of a jagged cutout in a tank showing the inside of it filling.
As James mentioned there is a new buzz of "high performance HMI" that reduces the graphics. My personal opinion is that it makes sense, but if you were to put a non-graphic grayscale HMI in front of a marketing department, it would be thrown back to you in a second. I think there is a good balance of minimal graphics for appeal and easy understanding, but not to go overboard with colors and graphics because it then makes things worst.
