Question: How to set avatar of Mattermost integration bot?

We are running the Omnibus installation of Gitlab CE + Mattermost. Gitlab reports push events to our Mattermost channel with the correct user (the one that pushed). But the avatar shown next to the message in mattermost is the one of our Gitlab administrator! We like to instead have either the avatar of the pushing user, or at least a neutral avatar of the bot itself. How can we do that?