FeaturedImage.php

<?php
// Posts hasMany Images

// Posts hasOne FeaturedImages: where featured image is the first image for that post.

$this->hasOne('FeaturedImages', [
    'className' => 'Images',
    'foreignKey' => false,
    'conditions' => function ($exp, $query) {
        $subquery = $query
            ->connection()
            ->newQuery()
            ->select(['SubFeaturedImages.id'])
            ->from(['SubFeaturedImages' => 'images'])
            ->where(['SubFeaturedImages.post_id = Posts.id'])
            ->order(['SubFeaturedImages.id' => 'ASC'])
            ->limit(1);

        return $exp->add(['FeaturedImages.id' => $subquery]);
    }
]);

References:

http://stackoverflow.com/questions/30241975/loading-associated-model-data-in-cakephp3
http://stackoverflow.com/questions/29775766/using-limit-on-contained-model
cakephp/cakephp#14427