How long ago did you upload the image?
These are the best practices for uploading your gravatar:
(1) Prepare the image prior to uploading it and make sure it’s no larger than 128 pixels X 128 pixels square.
(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) Note that there is a time delay while your gravatar image propagates. If you are experiencing problems then go to the gravatar site http://en.gravatar.com/ and read the FAQs as it contains common questions and answers to them http://en.gravatar.com/site/faq/