Question: What is the average number of movies added to the lists of user 8516503? Give the user profile image URL on Mubi.
SQL: SELECT AVG(T1.list_movie_number), T2.user_avatar_image_url FROM lists AS T1 INNER JOIN lists_users AS T2 ON T1.list_id = T2.list_id AND T1.user_id = T2.user_id WHERE T2.user_id = 8516503