<?php

$string ='<img src="image.jpg" alt=212><img src="http://u...content-available-to-author-only...a.org/wikipedia/commons/e/e5/Raster_to_Vector_Mechanical_Example.jpg" alt=45>';
$pattern = '/(<img src=\")(?!http)/i';
$content = preg_replace($pattern, '$1NEW!', $string);

echo $content;
