Require basic community integartion plugin knowlige.

Some components need to tell Touch whare user avatar is stored. Those are usually components that handle user registration process and allow avatar upload. Like Mighty Registration. All you need is to add to CIP (Community Integration PLugin) one onRenderAvatar event

function onRenderAvatar(&$user)
{
    $user->avatar = "images/avatars/".$user->get('id').'jpg';        
}

This event simple set new property for user object.

Directory should start from Joomla root without slash at the begining.

Note touch does not generate avatar <img> elements with full path to image. Avatar is always displayed through index2.php file and may be resized to any size. So you do not need to worry about avarat size.