Code:
img.setAttribute("src", "images/"+image_name+".png");
        if( img.addEventListener )
        {
          img.addEventListener('click',
          function ()
          {
            if (this.className == "")
                this.className = this.className.replace("", "semitrans");
            else
                this.className = this.className.replace("semitrans", "");
            already_exist = false;
            
            for(id in markers)
            {
                if (markers[id].type == pointData.type && markers[id].sport == pointData.sport)
                {
                    if (markers[id].visible)
                    {
                        markers[id].hide();
                        this.setAttribute("src", "images/gray-"+markers[id].type+".png");
                        //document.getElementById(image_id).style.opacity = 0.4;
                    }
                    else
                    {
                        markers[id].show();
                        this.setAttribute("src", "images/"+markers[id].imagename+".png");
                        //document.getElementById(image_id).style.opacity = 1;
                    }
                    
                    already_exist = true;
                }
            }
            //if (already_exist)
                return false;
          }
          ,false);
        }
        else if( img.attachEvent )
        {
          img.attachEvent('onclick',function(e)
          {
            if (img.className == "")
                img.className = img.className.replace("", "semitrans");
            else
                img.className = img.className.replace("semitrans", "");
            already_exist = false;
            
            for(id in markers)
            {
                if (markers[id].type == pointData.type && markers[id].sport == pointData.sport)
                {
                    if (markers[id].visible)
                    {
                        markers[id].hide();
                        img.src = "images/gray-"+markers[id].type+".png";
                        //img.setAttribute("src", "images/gray-"+markers[id].type+".png");
                        //document.getElementById(image_id).style.opacity = 0.4;
                    }
                    else
                    {
                        markers[id].show();
                        img.src = "images/"+markers[id].imagename+".png";
                        //img.setAttribute("src", "images/"+markers[id].imagename+".png");
                        //document.getElementById(image_id).style.opacity = 1;
                    }
                    
                    already_exist = true;
                }
            }
            //if (already_exist)
                return false;
          });
        }