function my_default_get_group_avatar($avatar) {
	global $bp, $groups_template;

	if( strpos($avatar,'group-avatars') ) {
		return $avatar;
	}
	else {
		$custom_avatar = 'YOUR LINK TO YOUR AVATAR HERE';

		if($bp->current_action == "" || $bp->current_action == "my-groups" || $bp->current_action == "invites" ) {
			if ( $bp->current_component == BP_FORUMS_SLUG ) {
				$width = "20";
				$height = "20";
			}
			else {
				$width = BP_AVATAR_THUMB_WIDTH;
				$height = BP_AVATAR_THUMB_HEIGHT;
			}

			return '<img width="'.$width.'" height="'.$height.'" src="'.$custom_avatar.'" class="avatar" alt="" />';
		}
		else
			return '<img width="'.BP_AVATAR_FULL_WIDTH.'" src="'.$custom_avatar.'" class="avatar" alt="' . attribute_escape( $groups_template->group->name ) . '" />';
	}
}
add_filter( 'bp_get_group_avatar', 'my_default_get_group_avatar');
add_filter( 'bp_get_the_topic_object_avatar', 'my_default_get_group_avatar');