The default player avatar is associated with your Facebook account. Once you link a Facebook account to your TG account, it will automatically use your current profile picture from Facebook as a player avatar

But in TG you can also use a character avatar, which will override your Facebook photo. To do that:

• Tap on your profile picture at the top left corner of the game screen
• Tap on the white edit icon on the right side of your nickname
• Then tap on the “SET YOUR AVATAR” button
• Choose any from the available character avatar pictures
• Save your choice
  
  Done!