Can't change image attribute src
function mytheme_preprocess_image(&amp;$variables) {
   if ($variables['style_name'] == 'slides') {
       $variables['attributes']['data-original'][] = file_create_url($variables['path']);
       $variables['attributes']['class'][] = 'lazy';
       $variables['path'] = '/sites/all/themes/mytheme/images/grey.png';
   }
}


Regards