Cycle decrement For does not work - ActionScript 2

I'm trying to make an image gallery with ActionScript 2. The forward button works perfectly, but the backward button does not work. This is the code of the back button:

on(release){
    var imgs:Array = new Array();
    imgs[1] = {img: ImgBox1};
    imgs[2] = {img: ImgBox2};
    imgs[3] = {img: ImgBox3};
    imgs[4] = {img: ImgBox4};
    imgs[5] = {img: ImgBox5};
    imgs[6] = {img: ImgBox6};
    imgs[7] = {img: ImgBox7};
    imgs[8] = {img: ImgBox8};
    imgs[9] = {img: ImgBox9};
  for(var i:Number = 9; i <= 1;i -= 1){
    if(_global.contador <= 1){
        return false;
    }
    if(_global.contador != i){
        _root.imgs[i].img._visible = false;
    }
    if(_global.contador == i){
        _root.imgs[i].img._visible = false;
        _root.imgs[i-1].img._visible = true;
        _global.contador = i-1;
        return false;
    }
  }
}

What could be the problem? It is assumed that by pressing the forward button, _global.contador increases by 1, and at the time of using the backward one it should decrease by 1 if it matches.