Modified version of Hans Möller's stitchWP.sh to specify different wallpaper image per monitor.

stitchWPmod.sh

#!/usr/bin/env bash
# 2018 Hans P. Möller <hmoller@gmail.com>
################################################################################
# This script is a workaround for the lack of support from pcfman-qt   
# for differenct wallpaper images for each display environment. pcmanfm-qt handle 
# all the monitors as only one big monitor. This script takes the configuration 
# from pcfman-qt and replicate it in all the monitors creating a big canvas 
# and stitching a different image to it. The steps are
# 1) Takes the pxfman-qt configuration
# 2) Reads information about the images passed as argument to this script.
# 2) It takes current display configuration from xrandr.
# 3) Create new wallpaper image joining specified images for each monitor
# 4) Updates the wallpaper with the created image
#
# To use it, specify any wallpaper as you usually do (to set a wallpaper mode),
# then run this script. 
# Usage: stitchWPmod.sh [wallpaperImage]
# You need to specify one wallpaper per monitor.
################################################################################
PROFILE="lxqt" #profile used when running pcmanfm-qt --desktop
wpNew="$HOME/.config/pcmanfm-qt/$PROFILE/stitchWP.jpg" #where wallpaper is saved
settingsFile="$HOME/.config/pcmanfm-qt/$PROFILE/settings.conf"
changeWP="pcmanfm-qt --set-wallpaper" #command to use to change the wallpaper

#########################################################################
#Get actual wallpaper configuration
#wpFile=$(grep 'Wallpaper=' $settingsFile)
#wpFile=${wpFile:10}

wpM=$(grep 'WallpaperMode=' $settingsFile)
wpM=${wpM:14}

wpBg=$(grep 'BgColor=' $settingsFile)
wpBg=${wpBg:8}

#if [ $wpFile == $wpNew ]; then
#	exit
#fi

if [ $# -eq 0 ]; then
	echo "No images provided, stop"
	exit
fi

wImage=()
hImage=()
for (( i=1; i <= "$#"; i++ )); do
	#Get dimensions of each image file specified in cmd line arguments
	imageFile="${!i}"
	imageInfo=$(identify "$imageFile" | grep -o '[1-9][0-9]*x[1-9][0-9]* ')
	IFS='x'
	j=0
	for STRING in $imageInfo; do
		case $j in 
			0)
				wImage[$i]=$STRING;;
			1)
				hImage[$i]=$STRING;;
		esac
		j=$((j + 1))
	done
done



for ((i=1; i <= $# ; i++)); do
	echo ${wImage[$i]}
done

case $wpM in
	tile)
		exit;;
	none)
		exit;;
esac

################################################################################
# Get current display configurator
# xrandr regex, + don't have to be escaped in grep.
XOUT=$(xrandr | grep -oe 'current [1-9][0-9]* x [1-9][0-9]*' -oe '[1-9][0-9]*x[1-9][0-9]*+[0-9]*+[0-9]*')
IFS=$'\n' #separate to lines with internal field separator
dispNumber=0
for LINE in $XOUT; do
	array[$dispNumber]=$LINE
	dispNumber=$((dispNumber + 1))
done
dispNumber=$((dispNumber - 1)) #number of displays used

#parse the total screen xinerama size.
IFS=' ' #separate to lines with internal field separator
i=0
for STRING in ${array[0]}; do
	case $i in 
		1)
			canvasW=$STRING;;
		3)
			canvasH=$STRING;;
	esac
	i=$((i + 1))
done

#Parse each screen size and position
for ((i=1; i <= $dispNumber ; i++)); do
	IFS='x'
	j=0
	for STRING in ${array[$i]}; do
		case $j in 
			0)
				w[$i]=$STRING
				;;
			1)
				IFS='+'
				k=0
				for CHUNK in $STRING; do
					case $k in 
						0)
							h[$i]=$CHUNK;;
						1)
							x[$i]=$CHUNK;;
						2)
							y[$i]=$CHUNK;;
					esac
					k=$((k + 1))
				done
		esac
		j=$((j + 1))
	done
done

case $wpM in
	stretch)
		newImage="convert -size "$canvasW"x"$canvasH" xc:"$wpBg""
		for ((i=1; i <= $dispNumber ; i++)); do
			newImage="$newImage -draw \"image over ${x[$i]},${y[$i]} ${w[$i]},${h[$i]} '"${!i}"'\""
		done
		newImage="$newImage $wpNew"
		echo $newImage
		eval $newImage
	;;
	fit)
	#mantain aspect ratio, fit the screen from the inside, 
	#could not cover the whole screen background color could be seen.
		newImage="convert -size "$canvasW"x"$canvasH" xc:"$wpBg" "
		for ((i=1; i <= $dispNumber ; i++)); do	
			wRatio[$i]=$((${w[$i]} * 100 / wImage[$i]))
			#echo ${wRatio[$i]}
			hRatio[$i]=$((${h[$i]} * 100 / hImage[$i]))
			#echo ${hRatio[$i]}
			
			if [ ${wRatio[$i]} -lt ${hRatio[$i]} ]; then #fit horizontal, space up and down.
				Y=$((hImage[$i] * wRatio[$i] / 100))
				Y=$((${h[$i]} - $Y))
				Y=$(($Y / 2 + y[i]))
				W=$((wImage[$i] * wRatio[$i] / 100))
				H=$((hImage[$i] * wRatio[$i] / 100))
				newImage="$newImage -draw \"image over ${x[$i]},$Y $W,$H '"${!i}"'\""
			else #fit vertical, space on sides
				X=$((wImage[$i] * hRatio[$i] / 100))
				X=$((${w[$i]} - $X))
				X=$(($X / 2 + x[i]))
				W=$((wImage[$i] * hRatio[$i] / 100))
				H=$((hImage[$i] * hRatio[$i] / 100))
				newImage="$newImage -draw \"image over $X,${y[$j]} $W,$H '"${!i}"'\""
			fi
		done
		newImage="$newImage $wpNew"
		echo $newImage
		eval $newImage
	;;
	center) #image is NOT scaled. If image is not cropped n+1 could superpose x<n+1 image
		#TODO
	;;
	zoom) #image is scaled maintaining ratio, MUST cover the whole screen, pieces of image could be of of screen; if image is not cropped n+1 could superpose x<n+1 image
		IFS=' '
		newImage="convert -respect-parenthesis -size "$canvasW"x"$canvasH" xc:"$wpBg" "
		for ((i=1; i <= $dispNumber ; i++)); do	
			#echo "i=$i, !i=${!i}, w[i]=${w[$i]}, h[i]=${h[$i]}, x[i]=${x[$i]}, y[i]=${y[$i]}"
			newImage="$newImage \( '"${!i}"' -resize ${w[$i]}x${h[$i]}^ -gravity center -extent ${w[$i]}x${h[$i]} \) -geometry +${x[$i]}+${y[$i]} -composite "
			#echo $newImage
		done
		newImage="$newImage $wpNew"
		echo $newImage
		eval $newImage
	;;
	#tile) #not needed
	#none) #not needed
esac

#Set another wallpaper to force update
prechangeWP="$changeWP \"$1\""
eval $prechangeWP

#Set New Wallpaper
changeWP="$changeWP $wpNew"
echo $changeWP
eval $changeWP