1600 x 1280
in order to get a picture with a 1280 vertical width you need to look for pictures or wallpapers that originally are 1600 x 1280. then use most image editing programs to crop the image to 800 x 1280 pixels - width and height.
i for one prefer for my wallpaper to remain still while my icons are being swiped by. I also find most images i used have a center focus.
also i try to if possible to edit the bit rate to be 24 bit for the image. don't know if this helps to display the image better but since the note is reported to display at 24 bits why have images any larger.