in a custom javascript button, I have an animated gif (ajax-loader.gif
A colleague wrote this js file for me (AnimatedShowHide.js)
==========================
var j$ = jQuery.noConflict();

var AnimatedImageAppender = (function () {

    var imgId = "img_" + Date.now();

    return {

        ImagePath: 'foo',

  AddAnimation: function () {
            var top = j$(window).height() / 2 - 50;
            var left = j$(window).width() / 2 - 50;
            var positionPart = "position: absolute; top:" + top + "px; left: " + left + "px;";
            var image = j$("<img style='display:block; " + positionPart + "' id='" + imgId + "' src='" + this.ImagePath + "'/>").appendTo(document.body);
        },


        RemoveAnimation: function () {
            var selector = "#" + imgId;
            j$(selector).remove();
        }
    };

}());
=================
I added both to my static resources as private resources

here is my button code (I want to show the animated gif while the apex convert class is running and hide it on completion)
====================
{!REQUIRESCRIPT("/soap/ajax/23.0/connection.js")}
{!REQUIRESCRIPT("/soap/ajax/23.0/apex.js")}
{!REQUIRESCRIPT("https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js")}
{!REQUIRESCRIPT("/resource/AnimationShowHide")}

try{

AnimatedImageAppender.ImagePath = "https://www.berneckers.com/images/ajax-loader2.gif";
AnimatedImageAppender.AddAnimation();

var newurl= sforce.apex.execute("ConvertLead", "doConvert",{leadID : "{!Lead.Id}"});

AnimatedImageAppender.RemoveAnimation();

newurl1 = String(newurl);
if(newurl1.indexOf('error')>-1 )
  {
    alert(newurl);
    document.location.reload(true);
  }
else
  {
     parent.location.href = newurl;  // at completion, refresh to Opportunity page
  }
}
}
catch(err) {
   txt="There was an error on this page.\n\n";
   txt+="Error description: " + err.description + "\n\n";
   txt+="Click OK to continue.\n\n";
   alert(txt);
   document.location.reload(true);
}
============
when I click the button, the class completes successfully but I see no gif indicating "in progress"

help please