the code snippet will work on your page for all images. Even on that graphics page.
But if you create these images in a different way – and i see some Enfold Lightbox on paintings page we had to find then a different solution.
But if you use the same way like on mixed media and graphics page – the code will work there too.

But as mentioned – it would be nice to have a fallback for the bottom-bar title on lightbox. my advise – and not only for seo purpose – give an alt tag to your images via media library.

and use that snippet with fallback alt -tag as title inside bottom-bar:

Edit
this checks if original image is present and if widget-text or alt is present :

function mixed_media_lightbox() {
?>
<script type="text/javascript">
window.addEventListener("DOMContentLoaded", function () { 
(function($){
    $('.widget_media_image').magnificPopup({
      delegate: 'img',
      type: 'image',
      removalDelay: 500,  // to allow animated closing in ms
      mainClass: 'avia-popup mfp-zoom-in mfp-image-loaded mixed-media',  
      midClick: true,
      gallery: { 
        enabled:true,
        preload: [1, 1] ,
      },
      image: {
        titleSrc: false,
        markup: '<div class="mfp-figure">'+
                  '<div class="mfp-close"></div>'+
                  '<div class="mfp-img"></div>'+
                    '<div class="mfp-bottom-bar">'+
                      '<div class="mfp-title"></div>'+
                  //  '<div class="mfp-counter"></div>'+
                    '</div>'+
                '</div>',
        },  
        callbacks: {
          elementParse: function(item) {
            var originalSrc = item.el.attr('src');
            var fullSizeSrc = originalSrc.replace(/-\d+x\d+\./, '.');

            // Create a temporary image object to check whether the full resolution exists
            var img = new Image();
            img.onload = function() {
              item.src = fullSizeSrc;
            };
            img.onerror = function() {
              item.src = originalSrc;
            };
            img.src = fullSizeSrc;
            item.src = originalSrc;
          },

          markupParse: function (template, values, item) {
            var title = ''; // Initialize title as an empty string. This is our default fallback.

            // 1. Highest Priority: Check for content from a nearby .widget_text element
            var textWidget = item.el.closest('.panel-grid-cell').find('.widget_text');
            if (textWidget.length && textWidget.html().trim() !== '') {
              title = textWidget.html();
            } else {
              // 2. Second Priority: If no text widget, check the image's alt attribute
              var altAttribute = item.el.attr('alt');
              if (altAttribute && altAttribute.trim() !== '') { // Checks if exists AND has non-empty content
                title = altAttribute;
              } else {
                // 3. Third Priority: If no alt, check the image's title attribute
                var titleAttribute = item.el.attr('title');
                if (titleAttribute && titleAttribute.trim() !== '') { // Checks if exists AND has non-empty content
                  title = titleAttribute;
                }
              }
            }

            // Finally, assign the title to values.title, wrapping it in <h3> ONLY if there's content
            if (title) {
              values.title = '<h3 class="heading">' + title + '</h3>';
            } else {
              values.title = ''; // If no title was found at all, ensure nothing is displayed
            }
          },
        },
    });
})(jQuery);
});
</script>
<?php
}
add_action('wp_footer', 'mixed_media_lightbox', 9999);

And

#top .mixed-media .mfp-image-holder .mfp-content {
  max-width: 1200px; 
  box-sizing: border-box; 
  padding-bottom: 90px; 
  vertical-align: middle; 
}

#top .mixed-media .mfp-bottom-bar {
  margin-top: -36px;
  position: absolute;
  top: 100%;
  left: 0;
  width: 100%;
  cursor: auto;
  background-color: rgba(0,0,0,0.3);
  backdrop-filter: blur(4px);
  padding: 5px 10px;
}

#top .mixed-media .mfp-bottom-bar .mfp-counter {
  right: 5px
}

#top .mixed-media .mfp-content img.mfp-img {
  width: auto;
  max-width: 100%;
  max-height: 90vh !important;
  height: auto;
  display: block;
  line-height: 0;
  box-sizing: border-box;
  padding: 40px 0 40px;
  margin: 0 auto;
}

#top .mixed-media .mfp-content .mfp-title .heading * {
  font-weight: 400 !important;
}

#top .mixed-media .mfp-content .mfp-title .heading p {
  margin: 0.3em 0;
}

#top .widget_media_image img {
  cursor: pointer;
}

Now – what you have to do. : not all your images do have that widget_text
See Paintings Page. Or you have tried to do it with enfold lightbox implementation there. ?

the script is working – but the shown title is not allways defined. the reason – see above.

Best would be to set an alt or title tag as fallback to all of your images. For SEO purpose it might be better to use the alt tag.
then you had to change the else condition to:

values.title = '<h3 class="heading">' + item.el.attr('alt') + '</h3>';