PHP Image Resizer is a short script which resized external images via a url request system. It uses GD for the resizing. At the moment, it only supports GIF, JPEG and PNG.
Make sure you have GD installed and then you're good to go. Put the script on your php server and then request it like so:
http://mysite.com/resizer.php- Base URL?img=- parameter for the image URL (e.g. http://images.com/image.jpg)&w=- output width (in pixels)&h=- output height (also in pixels)
You can then use the url in img tags like so: <img src="http://mysite.com/resizer.php?img=http://images.com/image.jpg&w=100&h=100">