Ask HN: What do you think of implementing Gravatars in a social site?

I am developing a new social site, which is going to go live in February or March. (if you are curious it is: http://www.chilledlime.com) Now, I have to make a decision on how to get avatars. Should users be able to upload an image? Or should I let them link to an external image? Or the third option: Gravatars? What do you think? Is it OK to use Gravatars or maybe I should use something that is more customizable?