Podrías usar
expresiones regulares o usar por ejemplo la clase
DOMDocument:
Código PHP:
Ver original<?php
$html = '<h1>ladlfkajdlf asdflja sdfjla sdf dsf</h1>
adf
as <img src="http://papermashup.com/wp-content/themes/arthemia/portfolio/images/mimitah.jpg" height="271" width="304" />
adsfakdflasdjfal dfad
<img src="http://papermashup.com/wp-content/themes/arthemia/portfolio/images/mi333mitah.jpg" height="271" width="304" />adfadfasdfadf
dafsdfasdf';
$dom = new DOMDocument();
$dom->loadHTML($html);
$img = $dom->getElementsByTagName('img');
echo $img->item(0)->attributes->getNamedItem('src')->nodeValue;
//Resultado: http://papermashup.com/wp-content/themes/arthemia/portfolio/images/mimitah.jpg
Saludos.