How can I change the circular photo on title bar of Mail.app? Every time I send an email, there is a circular photo associated with the message (top right). I cannot work out how to change this.

This is when I am using Apple's Mail.app on a MacBook running macOS Mojave 10.14.5. The image I cannot change is not my Google photo identity. This is an @gmail.com email ID.
Try re-adding the Gmail account in Mail.app. This should resolve the issue.