So Yes, there is the tutorial how to make an image array. So if the character starts with 100 HP, make 100 images until the gauge is empty. BUT...what if your character levels up and in level 2, his max HP is 110. What would you do? Suggestions?
Look at this question I asked, the answer should be what you need.
http://answers.unity3d.com/questions/7449/creating-a-circular-progressbar-timer