You need to check the value that your passing to implode. It should be an array of image IDs. The code looks right. The shortcode must have the list of IDs or is just shows all images attached to the post.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users
The most recent ACF Chat Friday featured a live demo of how to register CPTs directly in the plugin, one of our most requested features. Check out the summary below for a replay of the demo, and don’t forget to register for the next session! https://t.co/k2KQ3WWBAz