How to make default avatars and make sure nobody changes there avatar I want to set them an avatar with my API system

how do I do that I need to know

The best way will be to this and this and probably the next 3 things you’ll ask is to use DiscourseConnect - Official Single-Sign-On for Discourse (sso) and set discourse connect overrides avatar (there are other overrides for other user fields).

You may also want/need to uncheck the site setting for allow uploaded avatars.