See: http://www.en.joomgallery.net/faq/general-faq/show-image-description-in-slimboxthickbox.html
On Community builder pages (and possibly all category views popups) :
In components/com_joomgallery/interface.php change this line:
$link .= '" title="'.htmlspecialchars($obj->imgtitle, ENT_COMPAT, 'UTF-8');
to the following:
$link .= '" title="'.htmlspecialchars($obj->imgtitle, ENT_COMPAT, 'UTF-8') .htmlspecialchars($obj->imgtext, ENT_COMPAT, 'UTF-8');
And also this one
$link .= '" title="'.htmlspecialchars($img->imgtitle, ENT_COMPAT, 'UTF-8');
to the following:
$link .= '" title="'.htmlspecialchars($img->imgtitle, ENT_COMPAT, 'UTF-8') .htmlspecialchars($img->imgtext, ENT_COMPAT, 'UTF-8');
[Subtle difference between the two : in 1st case the object is referred to as $obj and in the second as $img]