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):
Thanks for signing up for Jira Ops! I’m Matt Ryall, leader for the Jira Ops product team at Atlassian. Since this is a brand new product, we’ll be delivering improvements quickly and sharing updates...
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