<?php
$content = <<<'STR'
<img class="leftimg" 
src="https://example.ru/img/shema/2016-05-20%2013-58-37.jpg" 
alt="Забор из профнастила" width="550" height="309" />
STR;
 
$content = preg_replace('!<img(.*?)src(.*?)alt="(.*?)" width(.*?)height(.*?)\s\/>!si',
'<span itemscope itemtype="https://schema.org/ImageObject">
<span itemprop="name" style="display:none;">\\3</span>
<span itemprop="author" style="display:none;">Author</span>
<img itemprop="url contentUrl" \\1 src\\2 alt="\\3" title="\\3" width\\4 height\\5 />
<meta itemprop="width" content\\4>
<meta itemprop="height" content\\5></span>',$content);
 
echo $content;