Hi,
This usually happens when your images don't match your screen's aspect ratio. The only way to determine which pictures will appear centered is to check them individually and choose those with the main subject near the middle of the image. When using Fill in Settings > Personalization > Background > Choose a fit for your desktop image under Slideshow, Windows may crop images with the subject closer to the left or right edge, making them appear off-center.
If you want all your images to remain centered, you can select Center as this option shows the image at its original size. However, this may leave empty spaces around the images if they are smaller than your screen.