Check if image exists and compare file size

check2updated.php

<?php
stream_context_set_default( ['http' => ['method' => 'HEAD'] ] );
$head = array_change_key_case( get_headers( $fullremotefilename, 1 ) );
$content_length = isset($head['content-length']) ? (int)( $head['content-length'] ) : 0;
if ( $content_length ) {
   $img_size = @filesize( $localfilename );
   if ( $img_size !== false && $img_size !== $content_length ) {
      // images match!
   } else {
      // images don't match
   }
}
// don't forget to reset default stream settings
stream_context_set_default( ['http' => ['method' => 'GET'] ] );