To add an additional email address to your Gravatar account, first, log into Gravatar and visit https://en.gravatar.com/emails/ and click "Add Email Address". From there, enter the address you'd like to add and then check your email to confirm.
Once you've confirmed via the link in the confirmation email you received, you'll be taken back to the list of emails associated with your account where you can click on the new address just added and then select an image from the section below.