jQuery gallery with imageFUpload
You've got several issues here: 


PHP runs on the server, JavaScript runs on the client's browser so they don't know about eachothers variables. 
You're not doing anything with your $.get(..) results, you need to do something with the results of the ajax get request in a success or complete callback. 
You're calling a URL to a full page, i'm assuming you want just the image.


If your image names follow the naming convention I see where the thumbnails use /230x130/ in the path and the larger image uses /imba/ instead, then use the code below instead of your current click event:

slideShowIndex = 0;

$('#big-image-slider img.imagecache-imba').load(function(){
  hideSlideLoader();
});


$('.small-images a').click(function(sender){
  sender.preventDefault();
  showSlideLoader();
  var lnk = $(this);
  slideShowIndex = $('.small-images a').index(lnk);
  var imgUrl = lnk.find('img').attr('src').replace('/230x130/','/imba/');
  $('#big-image-slider img.imagecache-imba').attr('src', imgUrl);
  return false;
});

function showSlideLoader() {
  //$('#big-image-slider').html('&lt;img class="loader" src="/sites/all/themes/antisunna/img/ajax-loader.gif" /&gt;') 
  var loader = $('#big-image-slider .loader');
  if(!loader.length) {
    console.log('creating loader');
    loader = $('&lt;img class="loader" src="/sites/all/themes/antisunna/img/ajax-loader.gif" /&gt;')
    $('#big-image-slider').append(loader);
  }
  $('#big-image-slider .imagecache-imba').hide();
  loader.show();
}

function hideSlideLoader() {
  $('#big-image-slider .loader').hide();
  $('#big-image-slider .imagecache-imba').show();  

}

$('.left-arrow').click(function(){
  if(slideShowIndex &gt; 0) {
    moveSlideIndex(slideShowIndex--);
  }  
});


$('.right-arrow').click(function(){
  if(slideShowIndex &lt; ($('.small-images a').length - 1)) {
    moveSlideIndex(slideShowIndex++);
  }  
});


function moveSlideIndex(index) {
  $('.small-images a:eq(' + slideShowIndex + ')').click();
}