To give your avatar the right setting, you can easily change the background of standard avatars in the editor.
If you're uploading a photo-based (standard) avatar, click the scissors icon to remove the original background. Then, click the Background button to choose from dozens of built-in options — or upload your own custom image.
If you're using an existing video-based Premium avatar, just click the Background button and either select a preset or upload your own background image to place behind the avatar.