Latest Post Thumbnail Alt Text Missing

You need to define $image_alt and $image_title. I don’t see where you did it in your code.

You need to change/add like this

<?php if ( has_post_thumbnail() ) {
      $image_src = wp_get_attachment_image_src( get_post_thumbnail_id(),’thumbnail’ );
      $attachment_meta = get_post(get_post_thumbnail_id());
      echo '<img width="100%" src="'.$image_src[0].'" alt="'.$attachment_meta->post_excerpt.'" title="'.$attachment_meta->post_title.'">';
      } else { ?>
        <img src="<?php bloginfo('template_directory'); ?>/img/fallback.jpg" class="img-responsive" />
    <?php } ?>

Image’s ‘alt’ text is stored as a string in wp_postmeta with the meta_key of ‘_wp_attachment_image_alt’.

That you can grab like this

$img_alt =  get_post_meta($thumb_id, '_wp_attachment_image_alt', true);

EDIT:

<?php if ( has_post_thumbnail() ) {
      $image_src = wp_get_attachment_image_src( get_post_thumbnail_id(),'thumbnail' );
      $attachment_meta = get_post(get_post_thumbnail_id());
      $img_alt =  get_post_meta(get_post_thumbnail_id(), '_wp_attachment_image_alt', true);
      echo '<img width="100%" src="'.$image_src[0].'" alt="'.$img_alt.'" title="'.$attachment_meta->post_title.'">';
      } else { ?>
        <img src="<?php bloginfo('template_directory'); ?>/img/fallback.jpg" class="img-responsive" />
    <?php } ?>

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