Mark, the image storage location will depend on if safe_mode is enabled or not. If safe_mode is enabled then the images will all go into a gallery/default folder, since php won't be able to create new folders.
If safe_mode isn't enabled, then there will be subdirectories created under the gallery folder. The subdirectory will be the forum id.
Under that there are 3 more subdirectories, full/medium/thumbs to store the fullsize, mediumsize, and thumbnail images.
So the directory structure with safe_mode disabled would look something like:
