An excellent chapter on this web service can be found in Sams Microsoft SharePoint 2007 Development Unleashed
It appears that you cannot filter a Picture Library by Folders using the QueryOptions parameter for .GetListItems(). I consider this to be a bug in the MOSS Web Service querying engine.
You can download a very handy tool from here to demonstrate the issue easily http://www.u2u.info/Blogs/karine/Lists/Posts/Post.aspx?List=d35935e0%2D8c0e%2D4176%2Da7e8%2D2ee90b3c8e5a&ID=12
To reproduce the bug:
- Make a photo library with 2 subfolders
- Make a normal document library with 2 subfolders
- Adding several photos to this photo library
- Adding several photos to the document library
- Opening up the tool, right-clicking on your list and selecting "GetItems" from the context menu
- Go to the View Options tab and entering your document path (e.g. MyListName/MySubfolderName)
- You will get the whole list from the picture library regardless of this parameter..... BUG!
- Try the same thing on the document library - and the Folder option works as expected.
In particular, this CAML will not work on a Photo Library where my List Name is 'PhotoLibrary' and my subfolder is '1':