I just added the following lines to the beginning of Slide.getPartDataInputStream():
<pre>if (part.getData() != null)
      return new ByteArrayInputStream(part.getData());</pre>
It works, so the thumbnail does not have to be rotated anymore, as it is created from the downscaled image.
But I'm really not sure, if this could break something else, as I don't know the code well enough... Maybe the author of this method can say something about possible side effects of this modification?