Just put in this movement script in the step event of the object and the sprite will automaticaly mirror itself depending on which direction it is moving.
if keyboard_check_pressed(vk_left)
{
motion_set(180,4);
image_xscale=-1;
}
if keyboard_check_pressed(vk_right){
motion_set(0,4);
image_xscale=1
}
if keyboard_check_released(vk_left)
{
motion_set(0,0);
}
if keyboard_check_released(vk_right)
{
motion_set(0,0);
}

P.S.
Thanks death-droid.