If you have saved avatar of that user using avatar service, then you have to call a separate method for fetching the avatar of that user please find the link
here for details..
Another option of saving and fetching the avatar of user along with score is to add additional information while saving the score and fetching that info while showing leaderboard . Please find the link of blog
for saving and fetching additional info with leaderboard .
Let me know if it helps.