How to resize automatically when users upload images by CCK image field
Imagecache is great, but since it keeps the original file around, it won't save space. In fact, if you have multiple presets, it will increase your disk usage as it creates its caches of images of those sizes.

What you may want to investigate is putting some logic into nodeapi() (or, if on Drupal7, its equivalent function) when saving your node that looks at the filepath of whatever image field you are concerned with and does some image manipulation on it if it is over a certain size and then saving it in place.