A rather nice SharePoint feature for standard image libraries (type 109) is that when uploading a picture, it will automatically create a thumbnail and a compressed, web friendly version of the picture in the _t and _w hidden folders. Unfortunately the MOSS WCM PublishingImages list is not an image library, but rather a document library. This is so because the published start and end date/time feature depends on both versioning and approval being on, and this requires a list of type document library.
Some WCM sites try to use AssetUploader.aspx, which is used in the ootb computed field "Thumbnail" in the list content type, to generate the thumbnail, but as anonymous users have no rights on the \_layouts\ folder for secured publishing sites, this is a no go. A solution is to code an event receiver for the list and create _t and _w versions for added images. The no-code solution is to upload separate thumbnail images in a separate folder, and apply a naming convention when displaying thumbnails for the images in e.g. content query web-parts (CQWP).