Solution to add alt text attribute to pop-ups with images for the Map Series StoryMap template. This assumes the layer with pop-up is in each web map within app

esri-mapseries-alt-text-enhancement

// this represents with "custom-scripts.js" file located within the "app" directory of appliation files
define(["dojo/topic"], function(topic) {
  /*
   * Custom Javascript to be executed while the application is initializing goes here
   */

    // The application is ready
    topic.subscribe("tpl-ready", function(){
      /*
      * Custom Javascript to be executed when the application is ready goes here
      */ 
    });   

    // After a map is loaded (when the map starts to render)
    topic.subscribe("story-loaded-map", function(){ 
        // webmap object
        const webmap = app.map;
        
        // map layers
        const mapLayers = webmap._layers; 
        
        // bridge layer - set variable within loop of map layers object
        // this represents the target layer present in each web map
        // there is a component of the name the same in each web map; 
        // but there are also some numbers as part of the layer name in each web map
        let bridgeLayer;

        // loop over map layers object to extract bridge layer
        for (const property in mapLayers) {
          // set map layer variable on correct layer
          if (property.includes('CountyBridgesCIP')) {
            bridgeLayer = mapLayers[property];  
          }
        }

        // bind click event to bridge layer
        bridgeLayer.on('click', function() {
          // use setTimeout due to delay of click event and pop-up opening
          setTimeout(
              function() {
                // starting alt text
                const startingAltText = 'a view of a road bridge.  The bridge is named ';
                // name of bridge
                const bridgeNameElement = document.querySelectorAll('div.esriPopupVisible div.mainSection > div.header')[0].innerHTML;
                // alt text to assign to image
                const altTextForImage = startingAltText.concat(bridgeNameElement);

                // image element to add alt text to
                const imageElement = document.querySelectorAll('div.esriPopupVisible div.esriViewPopup > div.mediaSection div.image img.esriPopupMediaImage')[0];

                // set alt text
                imageElement.setAttribute('alt', altTextForImage);       
              },
              500
            );   
        });
    
      }); // end "story-loaded-map" topic
     
  });