Size problem

This works for me, but it is problematic since it assumes allow_url_fopen is on for images that aren't in the local filesystem, and I took out the test for an existing class attribute on the image. I'm not sure why that was there, but I assume you had a reason. I'm posting this not as a solution, but as an idea toward a solution.

foreach ($elements as $element) {
  $a = $dom->createElement('a');
  $element->parentNode->insertBefore($a, $element);
  $a->appendChild($element);
  $src = $element->getAttribute('src');
  if (isset(parse_url($src)['host'])) {
    list($img_width, $img_height, $type, $attr) = getimagesize($src);
  } else {
    $path = urldecode(strtok($src,'?'));
    list($img_width, $img_height, $type, $attr) = getimagesize(DRUPAL_ROOT.$path);
  }
  if (($img_width=='')||($img_height=='')) {
    $img_width = $element->getAttribute('width');
    $img_height = $element->getAttribute('height');
  }
  $a->setAttribute('href', $element->getAttribute('src'));
  $a->setAttribute('class', 'photoswipe');
  $a->setAttribute('data-pswp-width', $img_width);
  $a->setAttribute('data-pswp-height', $img_height);
}

Looks like a project-specific issue. I think you need to disable this contrib module and use own text filter in custom code

No, I think you are missing the point of the problem. If an inline image is resized in the WYSIWYG editor, your code as it is picks up the height and width of the resized image from the html img attributes, not the intrinsic size of the image, so clicking on the image just brings up the same small version of the image in Photoswipe instead of the full-sized image, which is the point of using Photoswipe. What I have given you fixes that for both locally and remotely hosted images.

Consider this scenario: say I insert an inline image in a post that's 4000px x 3000px intrinsically, and I scale it down in the WYSIWYG editor so it is 200 x 150. A user clicks on the image expecting a larger version, but with your code all they get is the same 200 x 150 pixel scaled image, whereas with what I have done the image fills the window as expected and they can further zoom to the intrinsic size.

Thanks, now I can confirm it