search in includes/functions.php:
$thumb = "<img src=\"".$file_src."\" border=\"0\"".$width_height." alt=\"".format_text($image_name, 2)."\" />";
and replace:
$thumb = "<img src=\"".$file_src."\" border=\"0\"".$width_height." alt=\"".format_text($image_name, 2)."\" title=\"".format_text($image_name, 2)."\" />";
search:
$thumb = "<img src=\"".$file_src."\" border=\"".$config['image_border']."\"".$width_height." alt=\"".format_text($image_name, 2)."\" />";
and replace:
$thumb = "<img src=\"".$file_src."\" border=\"".$config['image_border']."\"".$width_height." alt=\"".format_text($image_name, 2)."\" title=\"".format_text($image_name, 2)."\" />";