Hey Carminie, I am new here but i have been programming for a few years. My suggestion is make a health bar image that has a set of animations that each are at a different point in the health bar. Then when you lose heath or gain health you can set the animation to a different frame. With this you can also incorporate an instance variable that represents the amount of health you have.
[|||||||||]- first frame
[|||||||| ] - second frame
[|||||| ] - third frame
..... - other frames
[ ] - empty health (dead)
there may be a better way or even a better way of explaining this.
sorry if this is not helpful.