You can set a profile picture by clicking the camera icon beside your username in the Play section. Pressing that should take you to your photo library and you can select the image you wish to use.

If nothing happens when you press the camera icon, you have denied the game access to your photo library (which it needs only to let you choose from your pictures). In that case, you need to follow these steps:

Android:

Go to your phone's settings -> Applications -> Application Manager -> Word Domination -> Permissions and toggle ON storage. After that, go back to the game and press the hexagon again.

iOS:

On iOS 11 permission is not needed to show the photo library.

Go to your phone's settings -> scroll down and click on the Word Domination application -> Toggle ON the Images permissions. After that go back to the game and press the hexagon again.