For me, it'd be cool if FotoBilder's 'full-size' images (and possibly also thumbnails) could reference graphics at arbitrary locations in my filesystem. I have a ton of pictures on my Windows box all nicely organised into directories which are handy for browsing locally, and in order to display them with FotoBilder I'm faced with their dumping all of my organisation in favour of relying on FotoBilder to do the organisation, or duplicating all of my data. On a UNIX system I could pull tricks with the database and symlinks, and I could do a similar thing with NTFS links on Win2k, but that's hardly clean.
What I would really like is the ability to make use of the FotoBilder interface to manage 'public' access to my photos while still keeping the pretty (but rigid and metadata-less) filesystem layout I have for my own organisation sakes. There is of course the problem of me moving images around and breaking references, but that's my own problem.
If I were to use it, I'd let FotoBilder handle the thumbnails as I don't habitually generate thumbnails of my images and store them with the full-size photos, but I guess some people might want to do that too for different reasons, so if that's easy it might be worth allowing also.
All of the ways I've thought of doing this involve storing references somewhere, where I've considered storing in the filesystem itself (sym/hardlinks, which would be a hack and would have to work differently on Windows if at all), storing in files (also a hack, but at least a cross-platform hack), or in the database which possibly involves complicating the database schema. I imagine it'd be popular functionality if it can be done, though.