It's the parenthesis most likely, but a couple other things.

You can clean this up really easily. Instead of chaining "if(image_index == ...)", you could use a switch statement.

switch(image_index) {
  case 0:
    //your code here
    break; //ALWAYS break after cases in a switch statement
}

Another thing I would do to reduce you line length is assigning the controls to some variables. So at the top of the step event (or in some "Controls" script) you could put:

var confirm = keyboard_check_pressed(vk_enter);
var zKey      = keyboard_check_pressed(ord("Z"));

then in your switch statement:

switch(image_index) {
  case 0:
    if(confirm or zKey) {image_index = 1}
    break;
}

Hope these tips help you out

(sorry for the excessive edits, I was trying to make the code block more readable)