The paperwhite is 758x1024, as documented in the Kindle User's Guide for the Paperwhite:
http://kindle.s3.amazonaws.com/Kindl...sers_Guide.pdf
Quote:
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Display — 6" diagonal XGA display, 758 x 1024 pixels, 212 dpi resolution, 16-level gray scale.
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And yes, although it also says XGA, it is 10 pixels narrower than a real 1024x768 XGA display.
To fill the screen with no borders, you need images of that size (758x1024).