gistfile1.txt

// example HTML:
// <img class="background-image" src="images/mobile/everydaycare.jpg">
// Description: Serve mobile image first, and swap directory if page is desktop

function swapImageMobileFirst() {
   var $bgImages = $('img.background-image');
   var loadCount = 0;
   
   $bgImages.each(function(){
      // Get image src
      var src = $(this).attr('src');
      
      // If mobile, use mobile image
      if (screenWidthValue === "mobile") {
         var res = str.replace("desktop", "mobile");
      }
      // If not mobile, use desktop image
      else {
         var res = str.replace("mobile", "desktop");
      }
      
      // Set new src
      $(this).attr('src',res);
      
      // On load, increase loadCount
      $(this).on('load',function(){
         loadCount++;
         //after all images are loaded, do stuff
         if (loadCount === $bgImages.length){
            postImageLoad();
            alert('all '+loadCount+' images are loaded.');
         }
      });
   });
}