This will do the job and will give you the latest image_src as your definitions shows the id in images table is AUTO_INCREMENT
so ORDER BY ii.id DESC
in group_concat will group the images in descending order and by using SUBSTRING_INDEX
will give you the latest image
SELECT i.*,
SUBSTRING_INDEX(GROUP_CONCAT( ii.image_src ORDER BY ii.id DESC ),',',1) image_src ,
SUBSTRING_INDEX(GROUP_CONCAT( ii.caption ORDER BY ii.id DESC ),',',1) caption
FROM itineraries AS i
LEFT OUTER JOIN itineraries_images AS ii ON i.id=ii.itineraries_id AND ii.is_live=1
WHERE i.is_live=1
GROUP BY i.id
ORDER BY i.id, ii.sort