This is something that I just did recently on a site and it might help you. In my case I wanted the images to be responsive in the same way that images added to the content area are made responsive, so I make WP do the work for me.
You’ll need to adjust this because I only work with image ID values, but it will give you the idea.
$image_id = get_field('YOUR IMAGE FIELD', false, false);
$image_size = 'YOUR IMAGE SIZE'; // set this to the larges image you want shown
$image = wp_get_attachment_image_src($image_id, $image_size);
$image_alt = get_post_meta($image_id, '_wp_attachment_image_alt', true);
if (!$image_alt) {
$image_alt = get_the_title($post->ID);
}
$image_tag = '<img src="'.$image[0].'" width="'.$image[1].
'" height="'.$image[2].'" alt="'.$image_alt.
'" class="size-'.$image_size.
' wp-image-'.$image_id.'" />';
$image_tag = wp_make_content_images_responsive($image_tag);
echo $image_tag;
The important part is setting the class of "wp-image-{$image_id}" which is all that WP uses to add the srcset.
