#potm {
  /* center vertically and horizontally */
  position: absolute;
  top: -9999px;
  bottom: -9999px;
  left: -9999px;
  right: -9999px;
  margin: auto;
}

#potm.hc {
  /* fill horizontally, crop */

  max-width: unset;
  max-height: unset;
  width: 100%;
  height: auto;
}

#potm.vc {
  /* fill vertically, crop */

  max-width: unset;
  max-height: unset;
  width: auto;
  height: 100%;
}

#potm.bs {
  /* fill both ways, shrink */

  max-width: 100%;
  max-height: 100%;
  width: auto;
  height: auto;
}