<?php
$content = '<img whatever title="my title" whatever src="mysrc.jpg" whatever />';

//replace title with alt
$replaced = preg_replace('/(\<img.*?)title/', '$1alt', $content);

//replace src with data-src
$replaced = preg_replace('/(\<img.*?)src/', '$1data-src', $replaced);

//add the rest
$replaced = preg_replace('/(\<img.*?)\/\>/', '$1'.' '.'class=\'l-img artcl-img\' src=\'load.png\' />', $replaced);

echo($replaced);
?>