Ok, ho risolto, posto la soluzione:
Va fatto override del file layouts/joomla/content/intro_image.php
nella posizione /*your_template*/html/layouts/joomla/content/intro_image.php
e aggiunto questo codice
<?php if (empty($images->image_intro)) : ?>
<?php $imgfloat = empty($images->float_intro) ? $params->get('float_intro') : $images->float_intro; ?>
<div class="pull-<?php echo htmlspecialchars($imgfloat, ENT_COMPAT, 'UTF-8'); ?> item-image">
<?php if ($params->get('link_titles') && $params->get('access-view')) : ?>
<a href="<?php echo JRoute::_(ContentHelperRoute::getArticleRoute($displayData->slug, $displayData->catid, $displayData->language)); ?>"><img
<?php if ($images->image_intro_caption) : ?>
<?php echo 'class="caption"' . ' title="' . htmlspecialchars($images->image_intro_caption) . '"'; ?>
<?php endif; ?>
src="images/blog/empty.jpg" alt="Foto assente" itemprop="thumbnailUrl"/>[/url]
<?php else : ?><img
<?php if ($images->image_intro_caption) : ?>
<?php echo 'class="caption"' . ' title="' . htmlspecialchars($images->image_intro_caption, ENT_COMPAT, 'UTF-8') . '"'; ?>
<?php endif; ?>
src="<?php echo htmlspecialchars($images->image_intro, ENT_COMPAT, 'UTF-8'); ?>" alt="<?php echo htmlspecialchars($images->image_intro_alt, ENT_COMPAT, 'UTF-8'); ?>" itemprop="thumbnailUrl"/>
<?php endif; ?>
<>
<?php endif; ?>
Dove nel src va messo il percorso della foto, e nell'alt la descrizione della foto:
src="images/blog/empty.jpg" alt="Foto assente"