How do I move the origin of a sprite?

I am trying to create a health bar. It's just a red sprite that changes it's width according to the players health. Problem is that the position of the health bar is set at it's center. When I change the width it gets shorter on the left and the right side.

How do I move the origin of the sprite to the top left so that the left side of the sprite always stays at the same place when I change the size of the sprite?

(I hope I was able to make clear what I mean, I am still lacking the correct terminology.)

you set that in the image editor. Image points is the second icon from the bottom on the left side of the window, right above collision polygon.

image point 0 is the origin.