I came across this website today while researching online graphical tools. It is a site that allows you to upload and then resize and modify a graphic, much in the way you would with a desktop application like GIMP or Photoshop. Although this web application is not quite advanced as something like GIMP, it does allow you to resize, apply filters and touch up photos and such.

I like these types of sites because I think it is really cool to see more and more highly functional applications work from the web browser, rather than having to download and install them.

http://gui.picresize.com/picresize2/

They also provide an API so you can use this type of functionality on your own website. You can submit pictures to their server for resizing and modification.

http://gui.picresize.com/api/

However they do impose some limits on usage. So if you are going to be doing a large amount dynamic graphic modification for a website, it might be best for you to just install a graphics library yourself. GD is a popular (and Open Source) graphical development toolkit and has implementations in most of the popular web languages, including PHP, Python, Perl and Ruby.

http://www.libgd.org