How to get post image root URL?

The get_attached_file() function returns the path to a file based on the attachment ID:

$FeaturedImage = get_attached_file( get_post_thumbnail_id() );

Getting the path to a specific size is more complicated. WordPress stores the filename for resized versions of the images in as attachment metadata, that can be retrieved with wp_get_attachment_metadata(). Once you have the filename of the resized version, you just need to replace the original filename in the path with the resized version’s filename:

$image_id   = get_post_thumbnail_id();
$image_meta = wp_get_attachment_metadata( $image_id );
$image_path = get_attached_file( $image_id );

if ( isset( $image_meta['sizes']['homepage-column1']['file'] ) ) {
    $image_path = str_replace( $image_meta['file'], $image_meta['sizes']['homepage-column1']['file'], $image_path );
}