For anyone using MySQL storage, I've been doing some reading and it seems the recommended approach is to use the MySQL database to store the shared metadata, but then use a local database/cache on each client for thumbnail storage (cover art, fan art etc.) .
As far as I can tell, this means that path substitution is generally frowned upon, so don't store thumb images on the network. XBMC will automatically grab thumbs from the internet as it needs them (ie. when viewed, or in the background) since the source URL (eg. "http://cf2.imgobject.com/t/p/w500/6sASq ... RecPD2.jpg
", or a network image location within your media library) will be held in the central MySQL database. The XBMC client will then store the resized images it grabs locally in its cache (the Thumbnails folder), and update the local Textures12 database with the image details thus mapping movies/tv shows/music/etc. to the cached images.
So when configuring a new XBMC client, all you need to do is:
1) Add the video (and optionally music) database settings to advancedsettings.xml
2) Optionally (to save a tiny amount of space) delete the local Database/MyVideos* (and optionally Database/MyMusic*) database(s) that are no longer required
I had thought that "sharing" a common network-based thumbnail folder would mean it was no longer necessary for each client to re-download each thumbnail image, but without the Textures database being present in MySQL (not supported - not sure if there any plans to support in future) this means each XBMC client has no knowledge of the network based thumbs.
One solution might be to sync each XBMC client with a "master" copy of Textures12.db but this isn't recommended and very hack-ish (though it might be a solution to get a new client "off the ground" with all images present, but probably not a long term solution).
One other reason not to use network-based thumbnails is that the design of XBMC in some places tends to assume local storage is being used, and can thus make a lot of "stat" calls (file information, directory walking etc.) which can, when occurring across a network SMB/NFS connection thanks to path substitution, incur an unintentionally significant overhead compared with local storage. Optimisations are being made to improve the disparity between local and network storage, but for now local would be the preferred choice if you want maximum performance.
So I've reverted back to local SD card-based thumbnail storage with my central MySQL video/music database, which is all fine as long as your SD card is big enough: my main Revo 3700 XBMC player with 514 movies, 34 tv shows/1538 episodes is running at just under 3.5GB thumbnail storage (all movies and tv shows have full cover art/fan art etc.).