Converts wallpaper picture to 1920x1080 dual monitor setup - left monitor filled with gray, right with input image. Uses ImageMagick.

prepare_wallpaper.sh

#!/bin/bash

if [ "$#" -ne 2 ]; then
	echo "$0 <input_file> <output_file>"
	exit 1
fi

convert "$1" -resize 1920x1080 -background "#222222" -gravity east -extent 3840x1080 "$2"