Because of performance issues with gravatar.com, they're cached and the cache is still being populated. If yours isn't there, just give it a bit of time.
Here's the FAQ entry I've proposed.
Where do the author's pictures come from and how do I set mine?
They come from the Gravatar associated with that author's cpan.org email address. To set yours, simply sign up with gravatar using your cpan.org address. We cache the gravatars so give it a day or so to catch up.