Use gd
to recreate the graphical part of the image in a new one, that you save with another name.
See PHP gd
edit 2017
Use the new Imagick feature.
Open Image:
<?php
$incoming_file="/Users/John/Desktop/file_loco.jpg";
$img = new Imagick(realpath($incoming_file));
Be sure to keep any ICC profile in the image
$profiles = $img->getImageProfiles("icc", true);
then strip image, and put the profile back if any
$img->stripImage();
if(!empty($profiles)) {
$img->profileImage("icc", $profiles['icc']);
}
Comes from this PHP page, see comment from Max Eremin down the page.