Get image max width - This will give us image’s max-width: 100% value in pixels and it’s supported in IE9, Chrome, Firefox, Safari and Opera. We can also take this further and add support for older browsers by loading the image into an in-memory object:

img_max_width.js

// Get image's max-width:100%; in pixels
function getMaxWidth(img) {
  var maxWidth;

  // Check if naturalWidth is supported
  if (img.naturalWidth !== undefined) {
    maxWidth = img.naturalWidth;

  // Not supported, use in-memory solution as fallback
  } else {
    var image = new Image();
    image.src = img.src;
    maxWidth = image.width;
  }

  // Return the max-width
  return maxWidth;
}