@Kaspar_L_Palgi, is correct. Avatar images still cannot be retrieved, set, or updated for contacts or companies through the APIs.
If you would like to see this functionality added, please upvote and comment on this feature request, following these best practices. If your use case is different than that idea, feel free to post a new request.
That said, HubSpot still tries to fill contact and company avatars from its proprietary database and via social media and other sites. Gravatars, for example, are often picked up for contacts while logos are picked up for companies. See this article for more information.