Try changing how the background image is displayed:
- Open Settings > Personalization > Background.
- Under the picture layout option, try Fill, Fit, or Center and see which one makes the picture look correct.
- Fill can crop parts of the image.
- Fit keeps the whole image visible.
- Center shows the image at its original size.
If the picture still looks wrong after connecting an external monitor, the display resolutions may be causing it. In that case:
- Open Start > Settings > System > Display.
- Select the external display.
- In Display resolution, choose a different resolution temporarily.
- When asked Keep these display settings?, select Revert.
Also, images with a different aspect ratio than the screen can be cropped or scaled. A 16:9 image usually fits best on most modern displays.