<?php
$icerik ="
- Selam naber <img id=\"blabla\" src=\"http://www.dogrulink.com/resim.jpg\" alt=\"Google\" test=\"BlaBla\"/>

- selam iyi sen ? 

<img id=\"blabla\" src=\"http://www.yanlislik.com/resim.jpg\" alt=\"Google\" test=\"BlaBla\"/>

- bye

<img id=\"blabla\" id=\"blabla\"  id=\"blabla\"  src=\"http://www.yanlislik.com/resim.jpg\" alt=\"Google\" test=\"BlaBla\"/>
";

preg_match("@<img(.*?)src=(.*?) @is",$icerik,$cikti);
echo str_ireplace(array('"',"'"),"",$cikti[2]); //Çıktı http://www.dogrulink.com/resim.jpg
?>