var i = 0;

var imageArr = [];

imageArr[0] = 'img/img_BMW.jpg';
imageArr[1] = 'img/img_Chr.jpg';
imageArr[2] = 'img/img_Mas.jpg';
imageArr[3] = 'img/img_Merc.jpg';

function imageCycle(){
  document.imgCycle.src = imageArr[i];

  if(i < imageArr.length - 1){
    i++
  }
  else{
    i = 0;
  }

  console.log(i);
}

function imagePrev(){
  document.imgCycle.src = imageArr[i];

  if(i == imageArr.length - 1){
    i = 3;
  }
  else{
    i--;
  }
}