Discourse supports configuring third-party avatar services, so you can easily integrate WeAvatar avatars.
To configure this, search for the keyword avatar in your admin settings, then adjust your settings to match what is shown in the image below.
The value for the final item external system avatars url is https://weavatar.com/avatar/?d=letter\u0026letter={first_letter}\u0026s=144. The avatar size is intentionally hardcoded here to prevent the avatar background color from changing when avatars are displayed in different sizes, though you can of course modify it to use the {size} parameter yourself.