To change the default user and group avatars in Bitbucket Server to your custom avatars, follow the steps listed below:

1. Disable Gravatar support in Bitbucket Server under Administration -> Accounts -> Avatars and uncheck "Enable Gravatar support" - (only for user avatar customisation)

2. Shut down Bitbucket Server

3. Rename the existing default-avatar-*.png images to default-avatar-*-original.png in <BITBUCKET_INST>/atlassian-bitbucket/static/images/avatar based on which avatars you wish to customise and replace them with your custom images

4. Start up Bitbucket Server

5. Invalidate the browser cache by pressing (Cmd+Shift+R) on the respective user or group profile screens in Bitbucket Server