By default, Avaya Workplace Client uses your initials as your avatar. However, your system administrator can configure it to be replaced with an avatar picture.
If you log in using Avaya Cloud Services or an SSO account (Gmail, Office365, Salesforce), it uses the avatar image from that account.
For Avaya Workplace Client on Windows, if configured for contacts from Microsoft Exchange using EWS, it can use your avatar image from Microsoft Exchange.
You cannot change the avatar image from within your Avaya Workplace Client. You need to update it through the source from which it is being obtained.