Images for the health bar are stored in Health/. Whenever you run out of health, a cross will appear in the middle of the health bar to indicate a loss.
It is recommended to keep the images the same size as background.png will define the dimensions of your healthbar, and foreground.png will be cropped to fit those dimensions. If foreground.png has smaller dimensions, the game will scale the image to fit the width. If the height is smaller than background.png after this, the game will scale up the image to match the level of the health bar, cropping out the excess width out of view.