wordpress REST-API upload image to ACF

ACF Image field takes the post->ID of the uploaded attachment. That’s what you should be passing to $request->get_param('image');

Here’s the code I’m using for a when a CPT is inserted via API. I’m sending a POST request with custom fields values. One of the fields is an image URL which is uploaded, attached to the CPT, marked as Featured and also set as an ACF field.

add_action("rest_insert_mycpt", function (\WP_Post $post, $request, $creating) {
    $metas = $request->get_param("meta");
    if (is_array($metas)) {
        foreach ($metas as $name => $value) {
            # Repeatable Field (extra code just in case it helps someone)
            if( $name == 'bookmarks' ) {
                $save = [];
                foreach( $value as $uri ) {
                    $save[] = array('url' => $uri );
                }
                update_field($name, $save, $post->ID);
            }
            # Image field
            elseif( $name == 'img' ) {
                # $value here is an URL
                $save = getImageURL($value, $post->ID, $metas['file_name']);
                update_field($name, $save, $post->ID);
            }
            # Other fields
            else update_field($name, $value, $post->ID);
        }
    }
}, 10, 3);

# Upload image from URL
# @return Attachment ID
# wordpress.stackexchange.com/a/50666
function getImageURL( $img_url, $post_id, $name ){
    if( !function_exists( 'wp_generate_attachment_data' ) ) 
        require_once(ABSPATH.'wp-admin/includes/image.php');
    # Upload file
    $extension = getExtension($img_url);
    $filename = "$name.$extension";
    $uploaddir = wp_upload_dir();
    $uploadfile = $uploaddir['path'] . "https://wordpress.stackexchange.com/" . $filename;
    $contents= file_get_contents($img_url);
    $savefile = fopen($uploadfile, 'w');
    fwrite($savefile, $contents);
    fclose($savefile);
    # Insert into library
    $wp_filetype = wp_check_filetype(basename($filename), null );
    $attachment = array(
        'post_mime_type' => $wp_filetype['type'],
        'post_title' => $filename,
        'post_content' => '',
        'post_status' => 'inherit',
        'post_parent' => $post_id
    );
    $attach_id = wp_insert_attachment( $attachment, $uploadfile );
    $imagenew = get_post( $attach_id );
    $fullsizepath = get_attached_file( $imagenew->ID );
    $attach_data = wp_generate_attachment_metadata( $attach_id, $fullsizepath );
    wp_update_attachment_metadata( $attach_id, $attach_data );
    # Set as Featured Image
    set_post_thumbnail( $post_id, $attach_id );
    # Generate sizes: wordpress.stackexchange.com/a/8085
    $image_sizes = get_intermediate_image_sizes();
    foreach ($image_sizes as $size){
        image_make_intermediate_size($uploadfile, $size['width'], $size['height'], false); 
    }
    return $attach_id;
}

# Get extension from string containing filename.ext
function getExtension($filename){
    $filename = explode('?', $filename)[0];
    return substr($filename,strrpos($filename,'.')+1);
}