<?php
$text = '<p>Lorem ipsum dolor sit amet, <strong>consectetur</strong> adipiscing elit.<p>
<p>%image1%</p>
<h2>Lorem ipsum</h2>
<p>Cum sociis natoque penatibus et magnis dis parturient montes.</p>
<p>%image2%</p>
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. </p>
<p>%image3%</p>
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. </p>';
echo preg_replace('|%(.+)%|', '<img src="$1">', $text );