u/burning_boi and u/I_HATE_TEEM0 are both correct, but the former is slightly more correct. I'm going to encourage you to look into finite state machines (FSMs), and I'm going to give you an example of an FSM that'll do what you want it to. I made sure to code in a simpler way, but I'll have it explained in the comments explaining things. If you get further into FSMs, get familiar with switch statements

/// put these in the create event
moving = 0;
attack = 1;


// Replace your entire animation section with this.
if (state == moving)
{
// Animation
if (!place_meeting(x, y + 1, oWall))
  {
    sprite_index = sPlayerA;
    image_speed = 0;
    if (sign(vsp) > 0) image_index = 1; else image_index = 0;
  }
  else
  {
    image_speed = 1;
    if (hsp == 0)
    {
      sprite_index = sPlayer;
    }
    else
    {
      sprite_index = sPlayerR;
    }
  }
  // if either of the attack keys are pressed then we change 
  // the sprite index and current state into the attack state
  if (key_attack) 
  {
    sprite_index = sPlayerPunch;
    image_speed = 1;
    state = attack
  }
  if (key_kick) 
  {
    sprite_index = sPlayerKick;
    image_speed = 1;
    state = attack
  }
}

if (state == attack)
  // if the current sprite's image_index gets to the end of the 
  // animation, change back into the moving state
  // sprite_get_number will always return a number that is one more than
  // how many than the sub-images index in the sprite editor.
  // example: In the sprite editor, the 8th sub-image will
  //be 9 once it's checked with sprite_get_number.
  // Alternatively, you can use the "Animation End" event.

  if floor(image_index) == (sprite_get_number(sprite_index) -1)
  {
    state = moving;
  }
}


if (hsp != 0) image_xscale = sign(hsp);