If part of your monitor is black you can try using the buttons on the front of the monitor to adjust the positions till it's centered. There might be an auto-center setting so check for it.
Or your monitor resolution setting may not be suitable for the wallpaer. Try and change it and see.
If it's a custom image the size may not suit the Desktop?
PowerToys has an Image Resizer that allows you to quickly resize any image to fit certain resolution setting like 1024 x 768 or 800 x 600.
If none of the above try updating your Video Card drivers with the latest driver form the manufacturer's website.
I have a wallpaper that is always off centre, to the bottom right. Even when i try and stretch it out. It happens with all wallpapers, so i know its not a problem with the wallpaper itself.

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