Unfortunately, for now, you are stuck with the stock icons as you cannot change that avatar.
This is already an open feature request for this (you can vote for it and watch it if you want to receive notifications when there is an update):
If you already heard about Smart Commits in Bitbucket, know that you just stumbled upon something even better (and smarter!): Genius Commits by Better DevOps Automation for Jira Data Center (+ Server...
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