First ensure that the user is assigned to a K2 user group. You can go to K2 users and check that. Then ensure that the K2 user group of that user has the appropriate permissions to create items. You can set the K2 user group permissions in the administration ( K2 -> User Groups ). Finally create an instance of the K2 User module. Publish it to the pages you need. This will provide a login form for users. When user signs in then it will display it's profile image along with a button to add an item.