Those little avatars / user pictures in Xcode are linked to your Mac Contacts. This is not a Git or SVN feature.
You can associate a Mac Contact with a committer in Xcode and make it show up under Source Control – History. If the contact has a picture and you associate that contact with a Git committer then Xcode will show it in History. Here’s how it works in Xcode 6:
- in Source Control – History, hover over the name of the committer
- click on that super teeny tiny disclosure triangle
- from the drop down menu, select Add to Contacts
- you can now create a new contact, including a picture, or “Choose existing” and pick a contact
- confirm your choice and the icon shows up in your project
Xcode isn’t clever enough to apply the same committer contact to all your projects so you need to repeat this process for each project.
In the same manner you can replace and update contacts.