I've written to Kent Teachers to recommend Microsoft's Image Resizer:
Mike Trodd says: "I also use a free piece of software for resizing pictures for my website (www.cecil-road.kent.sch.uk) called Pix Resizer. Really easy to use and give you good control over the file type and size. Also does batch resizing very quickly! Plus, it resaves the images so that your originals are not altered in any way!"
http://bluefive.pair.com/pixresizer.htm
Helen