When you join SketchClub you'll start with one of the avatars from the Avatars compo as your profile image. You can change this at any time to a different community avatar or any of your recently uploaded sketches.
First Browse SketchClub to access the online community. Then tap the person icon in the toolbar to go to your profile page. Next tap the link to Edit your profile. And finally you'll see the different avatars that are available to choose as your profile image.