In the Lobby, simply tap on your avatar in the top left corner. From there, tap on your profile picture again to bring up the Avatar Store.
You have the option to select one of the poker faces from the store, or you can use your Facebook Profile picture by selecting the FB icon.
If you would like to change your custom (Facebook) avatar, you need to change your profile picture on Facebook.
If you are currently playing as Guest, you need to merge your Guest account with Facebook to be able to use your Facebook profile picture as your avatar in the game. To merge the two accounts, just tap on the Facebook icon in the top right corner of the Lobby and click Connect Now.
How can I use a custom profile picture in the game?
You’ll have to play with your Facebook or Google+ account to use a custom profile picture as your avatar in the game.
Your Facebook or Google+ profile picture and first name will be automatically displayed in the game.
To use your Facebook avatar:
Install the game on Facebook.com or PlayWSOP.com and sign in with your Facebook account
To use your Google+ avatar:
Install the game on PlayWSOP.com and sign in with your Google account
(For the time being, Google+ login is only available on PlayWSOP.com and cannot be used on other platforms.)
If you are currently playing as a Guest, you need to link your Guest account to your Facebook account first to be able to use your profile picture in the game. To link the two, just launch your Guest account and click on the Facebook icon in the top right corner of the Lobby.