Currently, the light box script tries the title attribute for the Image Title first. Only if there is no title attribute (in the page source, not in the WP admin), it will fall back to the alt attribute.
There is currently no easy fix for this.
The only way (except modifying or deleting all image titles in the admin) would be to put a small javascript snippet in your footer widgets that modifies the title/alt attributes of any images on the page, just before the light box script runs.
Something like:
<script>
jQuery('img').each(function () {
$this = $(this);
if ($this.attr('alt')) {
$this.attr('title', $this.attr('alt'));
}
})
</script>
Try that in an HTML or CODE widget (not sure what is available in WPBakery)