Good idea (and with other versions working), but... Rated 3 out of 5 stars
... I wasn't able to make it work on FF 3.0.6 nightly together with (no matter how many) pictures in a local directory under WinXP SP3. Without function... I can not rate it on a really neutral base.
But a suggestion: Look for a different algorithm which chooses the pictures, since it needs way too much time to scan a directory with many pictures (several hundrets).
If you need an idea how to reach this I could suggest you a few ways. Furthermore I possibly could give you a few ideas for enhancements.
But try to find out why it doesn't work in FF 3.0.6 (since I - and I assume I'm not the only one... - do not use FF 3.1 or 3.2 because of a few add-on's which still don't work with these versions of FF). Without this issue in mind I would rate it with *****.