You can insert custom Buddypress group avatars if no avatars have been uploaded by the user to the group. This detail can help sculpture your Buddypress site towards a more nuanced design.

Alter the path to your image in $custom_avatar and you’re set to go.

To enhance on this, you could also add custom default user profile and group cover images.

bp-custom.php

function default_custom_get_group_avatar($avatar)
	{
	global $bp, $groups_template;
	if (strpos($avatar, 'group-avatars'))
		{
		return $avatar;
		}
	  else
		{
		$custom_avatar = get_stylesheet_directory_uri() . '/images/image.png'; //fill in the path to your image
		if ($bp->current_action == "") return '<img class="avatar" alt="' . attribute_escape($groups_template->group->name) . '" src="' . $custom_avatar . '" width="' . BP_AVATAR_FULL_WIDTH . '" height="' . BP_AVATAR_FULL_HEIGHT . '" />';
		  else return '<img class="avatar" alt="' . attribute_escape($groups_template->group->name) . '" src="' . $custom_avatar . '" width="' . BP_AVATAR_FULL_WIDTH . '" height="' . BP_AVATAR_FULL_HEIGHT . '"/>';
		}
	}

add_filter('bp_get_group_avatar', 'default_custom_get_group_avatar');