Windows XP automatically generates little thumbnails for images in a folder, and along with this, generates thumbs.db - which is simply a cache file so it doesn't have to trawl through every image each time you open the folder.

I've recently been saving large folders of images (50-100 photos per folder) on my home server, and noticed that thumbs.db wasn't being created when I opened the folder. The samba share had full write permissions, so there was no reason the file couldn't be created. With such large files, it would take a few minutes for windows to generate thumbnails everytime I tried to browse them over the network share.

The solution, it seems - is to add the following line into the [global] section of smb.conf:

nt acl support = no

This apparently tries to map windows user permissions to the samba share permissions when turned "on". I also found I had to explicitly define writable = yes on my shares once I'd disabled the acl support (whereas I didn't before).