This is more of a concept page than a tutorial. I am gonna talk about ways you can manipulate objects to make a better game.
Mirroring Mirroring an object allows you to look 2 different directions well using a single sprite. This means you don't need 20 different sprites to look around. This is done by changing the image_xscale and image_yscale values between 1 and -1.
Sprinting Want to add sprinting to a game? Increasing the speed is fine and dandy, but your legs will still look like you walking. This means you should increase the speed of the animation itself. This can be done by changing the image_index. Say running is image_index += 1. Now sprinting could be image_index += 2.
Rotating your head This is great for games where your head isn't directly connected to the body. You simply rotate the head to look at the mouse. Of course at certain point, you stop the rotation so your head doesn't look straight up.