Hi Jfl,
Have been thinking about the type of animation you're mentioning.
It sounds like comic book animation? (i.e. picture background, then timed dialog boxes appearing)
While you can probably use Salasaga for it, I'm thinking that it might still be a bit too rough around the edges.
Have you looked at Wink?
http://debugmode.com/wink/It's not open source, but it's free and has both Linux and Windows versions. People also say good things about it too.
(I have no first hand experience with it, so can't really compare directly)