The cover images from the login screen can easyly be changed. Just upload new images into the mediabase and configure the ids in the backend section of the preferences.xml.

<phenotype>
  <preferences>
    <section name="backend">
      <img_id_cover>0</img_id_cover>
      <img_id_cover_error>0</img_id_cover_error>
    </section>
  </preferences>
</phenotype>

To design your own covers you can use the following PSD layout for the start.