Desktop client with support for WP2.9 thumbnails?

Hi all – I’m looking for a desktop client with support for attaching a thumbnail to a post (not to be confused with inserting an image within a post). Does anyone know of any suitable software?

Background:
To make life easy for a client I’ve created a WP2.9 CMS style site where all they have to do to add content is attach a thumbnail image (via the new feature in WP 2.9) and type away. The theme takes care of what size the image should be displayed as well as where and how it will be displayed in different areas of the site.

My client is also a technophobe and I think the WP backend scares them a little even though I’ve simplified it considerably and given them nice big easy buttons ?? I’ve suggested they might find it easier to use a desktop client to add content to their site as the process is much more akin to sending an email which they are used to.

But to work with my theme my client would need to be able to attach WP2.9 thumbnails to their posts. Does anyone oknow of a client (Mac or PC) which supports this?

Many thanks
Stef