In This article you see the way to set a feature image as a Advance custom field field.

function acf_set_featured_image( $value, $post_id, $field  ){
    if($value != ''){
        //Add the value which is the image ID to the _thumbnail_id meta data for the current post
        add_post_meta($post_id, '_thumbnail_id', $value);
    return $value;

// acf/update_value/name={$field_name} - filter for a specific field based on it's name
add_filter('acf/update_value/name=cursusfoto', 'acf_set_featured_image', 10, 3);