When my character stops, how do I show the standing part of his walking animation?

V5,x

In the <no key> event, go to the code tab and drag over Set the value of a variable, have the variable name as image_single and the value as the static, standing image, e.g, 2.

In the <left> and <right> key events, drag over Set the value of a variable again, have the variable name as image_single and the value as -1 (to start the animation again).

~-oOo-~

V6.x

In the <no key> event, drag over Execute a piece of code an add:

image_speed = 0

image_index = <standing image number>

In the <left> and <right> key events, drag over Execute a piece of code an add:

image_speed = 1