You can also upload a new avatar through the Tapatalk app.
To do this in the app:
Go to the top left hand side where you will see three dots or lines.
Click on that and then click on your username at the top
That will take you to your profile page.
When in there, you should see a bubble where the avatar would be located and a blue/green icon of a camera.
Clicking on that will allow you to either post a picture straight from your library or you can take a brand new photo and post it directly.