Go to your profile settings page here.

Click on the link in the text "You can change your avatar at gravatar.com".

Create an account by completing their registration form here and then activate your account using the link you will receive from WordPress.com. After activating your account, you should see the following message displayed on the screen:

Click on the "Sign in to Gravatar" button and select "Add an Image" under the profile icon at the upper right corner of the page, as follows:

Click "Upload New" and upload an image saved on your local machine.