<php 

$cek_bakalim= rand(1, 4);

if( $cek_bakalim == 1 )
    echo '<meta property="og:image" content="http://site.com/fbresmi.jpg"/>';
else
    echo '<meta property="og:image" content="http://site.com/fbresmi' . $cek_bakalim . '.jpg"/>';

?>