How many screens are on this desk?
Enter the resolution the way your OS reports it. If a screen is rotated, that means the portrait numbers.
You trace the four corners of each screen. No text is read from the photo — it only supplies positions, so resolutions and diagonals still come from the fields above.
On macOS and Linux, set each file on its own display. On Windows use the span file with wallpaper style set to Span.