I have a joomla website with the virtuemart module.
What I have are images and thumbnails which are hosted on a other domain.
Virtuemart gives me 2 options:<
You can make use of imagecopyresampled function of PHP.
Sample program (source: php.net)
<?php
// The file
$filename = 'http://valplibrary.files.wordpress.com/2009/01/5b585d_merry-christmas-blue-style.jpg';
$percent = 0.5; // percentage of resize
// Content type
header('Content-type: image/jpeg');
// Get new dimensions
list($width, $height) = getimagesize($filename);
$new_width = $width * $percent;
$new_height = $height * $percent;
// Resample
$image_p = imagecreatetruecolor($new_width, $new_height);
$image = imagecreatefromjpeg($filename);
imagecopyresampled($image_p, $image, 0, 0, 0, 0, $new_width, $new_height, $width, $height);
// Output
imagejpeg($image_p, null, 100);
?>