How can I make my character 'flip' in Game Maker?

I want my character to 'flip' or 'mirror' when I press 'A' or 'S'. Thing is, I've placed the origin everywhere on the character and he seems to only face one direction.

Video showing what I see: https://www.youtube.com/watch?v=1zZEDpO0yAM&feature=youtu.be

(This video shows the character not 'mirroring' or 'flipping' properly.)

I have done hours of research into this, and I get the same results: "Make sure the sprite has the center origin."

How would I accomplish this?

So all you have to do is assigning a negative scale using image_xscale:
image_xscale = -1;

The documentation page for image_xscale actually shows you an example with different scaling factors.