Apple - Apple Replaced my Gmail Image

Apple did not change your Gmail profile picture. What is (likely) actually happening is the following:

When you add a Gmail account to Mail.app, you can choose to sync contacts with Google. (It might even do it by default. I am not sure because it has been years since I set it up.) If you do so, all of the contacts in Contacts (Address Book) on your Mac will be sent to Google, including the contact that represents you. If you did not set up a specific contact for yourself in Mac Contacts, Apple did so and for that contact, the picture would have been the one used for your Mac user account.

When the contacts synced to Google, the one with your name, email and the hawk picture was sent along with it. Google reads your contacts and displays the icon from your contacts, NOT your Gmail/Google+ profile picture, next to the Reply/Forward box.

To test this, confirm you have a contact in your Gmail account that has your email address in it, and that it has that picture. Delete that contact and the picture next to the Reply/Forward box should return to normal. You can prevent this going forward by changing the picture for your entry in Contacts on your Mac.

I had this happen when I set up Messages with my Google account on a new Mac, without using Apple Mail or Contacts; suddenly Gmail was the stock image the Mac had defaulted to.

The fix was to log into Gmail via the browser, go into its settings:

-- the gear menu, not the Google Account menu (which was still showing my usual Google profile picture), where I found the offending image in the My Picture section, and was able to change it.