$text = '"i":"https://mylink.com/2634/2634441_200x200.jpg","l';
$pattern = "|\"i\":\"https://[^\"]+\/|is";
$text = preg_replace($pattern, "\"i\":\"http://newlink.com/", $text);
echo $text;
"i":"http://newlink.com/2634441_200x200.jpg","l
"i":"http://newlink.com/2634/2634441_200x200.jpg","l