The screen dimensions of that model are 1448 x 1072 pixels, which is not exactly 4:3. You should use that size to fill the entire screen, or a multiple of that if you want to be able to pinch zoom photos to see more detail.
The normal Kindle reflowable book formats leave a margin around images so you need to use a fixed layout format that can place images to the edges of the screen.
One possibility is using the Kindle comic book format which you can create using
Kindle Comic Converter from a zipped file of JPEG images (cbz).