The "Midsized Cached Images" addon provides you a new tag for your templates (MIDSIZE_IMAGE), which is a path to a resized version of your original image. Images are resized on-the-fly and cached in a special folder (there's no server load after the first resizing).
Typical usage is to allow browsing through screen-sized versions of your photos, and allow optional acces to the hi-res picture. See http://bleutonique.free.fr for a live example.
Tested with Pixelpost 1.5, and should also work with previous versions.
Version 0.2 adds a "Clear cache" button in the admin panel.