Lately this has happened to several bloggers who have posted to the forum here and remarkably a short while after they posted their gravatars have appeared.
These are the best practices for uploading your gravatar:
(1) Choose an image that scales both up and down in size without losing definition. Prepare the image prior to uploading it and make sure it’s 512 x 512 pixels or larger so it will look the best in the widest range of uses.
(2) The image filetype has to be either PNG, GIF or JPG. Make sure the file name contains only letters and numbers – no special characters – and make sure all letters are in lowercase.
(3) Also make sure the image file name has an extension (Example – image.jpg)
(4) There is a time delay while your gravatar image propagates. If you are experiencing problems then I advise you read the FAQs as it contains common questions and answers to them http://en.gravatar.com/site/faq/ Provided none of those apply to you and two days has passed since you uploaded your gravatar then you can contact Staff http://en.support.wordpress.com/contact/