$string = '<img src="http://gvsubhu.com/subhu.jpg" /><img src="http://gvsubhu.com/subhu_1.jpg" />';
$result = preg_replace('/(<img[^>]+>(?:</img>)?)/i', '$1<br />', $string); <img src="http://gvsubhu.com/subhu.jpg" /><br /><img src="http://gvsubhu.com/subhu_1.jpg" /><br />