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;
}

or try :
https://kriesi.at/support/topic/entry-title-h3-in-a-portfolio/#post-1485701

this is the main crux:

elementParse: function(item) {
          item.src = item.el.attr('src').replace( /-\d*x\d*\./, '.' );
},

in the callback function. The image link comes now from the src attribute.

because of that extra class on mainClass you can individually set :
see final solution at: https://kriesi.at/support/topic/lightbox-issues-2/#post-1486042

not familiar with site origin – but with the magnificPopup script ;)

but you can have your own lightbox script for those pages – using the magnificPopup :

Edit: see final solution at: https://kriesi.at/support/topic/lightbox-issues-2/#post-1486042

your img tags have no title nor alt attributes – but we can use the following description.

You are still using an outdated version of Enfold there. You should update the theme to match the current WordPress version, which uses a newer jQuery. You might want to get help with this. Enfold 4.5.7 and 7.1.1 are quite different, so you may need to customize it further then. At the very least you should have a full backup of your site. I recommend updating as follows: https://kriesi.at/support/topic/some-hints-and-advice-to-update-enfold/#post-1056107