Regardless if you share your project in a shared team-workspace or directly through sharing links, with comments enabled anybody with access to your project can annotate directly on images. To place an annotation on an image open the "comments" of the creative. Then move your cursor to the spot you want to annotate and click to place the annotation pin. Then write and send your comment.
In order to comment through sharing links, a user has to create a soft-authentication (leave their name & email).