Apple Mail only shows avatars if recipients have added images to contacts. Alternatively, you can set up Apple Branded Mail, a proprietary Apple format that displays your logo as an avatar in the inbox of Apple Mail, or set up BIMI with a Verified Mark Certificate (Apple Mail does not support CMC).
Some inbox service providers or email clients (e.g. Thunderbird, Airmail, and Postbox) rely on Gravatar to display an image.You can set up a free Gravatar account, add your avatar, and verify your addresses you’re sending from to that account to have your avatar displayed.
Almost every email provider has its own way of adding a profile picture to an inbox.This means you can only:
Add your avatar to a real inbox, limiting it only to that provider
Send mail from the same address that you set the avatar on
The way around this is BIMI (Brand Indicators for Message Identification). It is much more difficult to obtain, but supports by nearly all providers and allows you to send from any address on that domain.