Change profile picture For Apple communities

According to this, you have to be level 3 before you can upload a custom avatar. Take a look.

Build your reputation and get rewarded in the community - Apple Support

Once you get to level 3, you will have the ability to upload an avatar using the same screen that you used to pick the avatar that you are using now. The upload option will be in the lower right corner.

You can upload your custom avatar (Be Yourself award for Level 3 — 500 points, users and above).

The option to use a custom profile picture will be available when you reach Level 3.

Build your reputation and get rewarded in the community - Apple Support