The forums are powered by WordPress (specifically the bbPress plugin), WordPress uses your Gravatar profile picture (https://en.gravatar.com/).
So if you want to change your profile picture on the forums you’ll need to create a Gravatar account (with the same email address that you signed up with here) and upload your profile picture there.
Just so you know – this will set your profile picture for all websites that use WordPress to power them, not just the Cipher Challange.
Hope that’s helpful!