Does someone know a good web service to resize images ? Either an open source (PHP/Python/Ruby) application, or a company providing a web service api.
PHP provides image resizing support using image magick. Smart Image Resizer is a PHP script ready for use. Look at http://shiftingpixel.com/2008/03/03/smart-image-resizer/
To build your own PHP app, look at http://www.php.net/manual/de/ref.image.php
So you're asking for a website?
http://www.picnik.com/ is what Flickr uses.
I've created a URL-based service for manipulating images - urlimg.com. It basically works by creating specially-crafted URLs which correspond to image processing commands. For example if you wanted to resize an image at mysite.com/profile.jpg, you could use urlimg.com and make your markup like:
<img src="http://urlimg.com/resize/50x50/mysite.com/profile.jpg" />
Make your own service at Utility Mill (http://utilitymill.com). Here's one that I wrote that adds a simulated gallery wrap - http://utilitymill.com/utility/Gallery_Wrap_Image
. Define your own interface, parameters, processing logic, and you get not only an interactive web service, but you also get a callable API.
Another alternative is the image processing web service at www.lightspun.com .