If you're looking for the "best" full-screen imaging I'd use the native screen resolution:
7" Kindle HD: 1280 x 800
8.9" Kindle HD: 1920 x1200
Lower resolution images will be upscaled, which can make them look soft. Try both Rikk's suggestion (1024 x 600) and mine, then you be the judge. If you can't see the difference go with the lower resolution to reduce storage space.