Sorry for the delay in responding to your question, MvGulik!
1) For now actually, they're cached indefinitely -- but that definitely will change! We do want to be a proper cache and purge and refresh old content; we just haven't had the time to implement that side of the equation yet.
2) Ideally yes, there would be some way to force or request a refresh, but we're not sure yet how we'll expose that. One possibility is indeed some kind of timeout, after which maybe simply re-submitting the URL would "check" if the image has changed.
Let us know if you have any other ideas or food for thought.