Applications > Pictures > Options > Select Standby Screen. If you hit Options again at this point, you can Select All or Unselect All if you have a lot of images and that will be easier. Otherwise just check off which ones you want to rotate or just one, then Done at the upper right.
Lately I've been using the cover of the current book I'm reading as the standby screen. I'll upload a better 600x800 image of it to the reader for that purpose, then just delete the image when I'm done with the book.
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