This Thunar Custom Action will create a watermark on an image, with customized text from a YAD entry form.

watermark.sh

#!/bin/sh
command -v convert >/dev/null 2>&1 || { echo >&2 "I require convert, but it's not installed. aborting!";exit 1;}
command -v identify >/dev/null 2>&1 || { echo >&2 "I require identify, but it's not installed. aborting!";exit 1;}
command -v bc >/dev/null 2>&1 || { echo >&2 "I require bc, but it's not installed. aborting!";exit 1;}
basedir="$(dirname "$(readlink -f "${1}")")"
cd "$basedir"
caption=$(yad --entry --editable --no-buttons --width 410 \
	--title="Please enter your caption and press enter:")
if [ -z "$caption" ]; then
	printf "no caption was selected, aborting!\n"
	exit 1
fi
printf "caption is $caption\n"
if [ ! -d "$basedir"/backups ]; then
	mkdir -p "$basedir"/backups
fi
while [ $# -gt 0 ]; do
	file="$1"
	if [ -s "$file" ]; then
		cp -f "$file" backups
		export imagesize=$(identify -format "%w,%h" "$file")
		export imagewidth=$(echo "$imagesize" | cut -f1 -d',')
		export imageheight=$(echo "$(echo "$imagesize" | cut -f2 -d',')*0.06000" | bc)
		convert -background '#0008' -fill white -gravity center \
		-size $(echo $imagewidth)x$(echo $imageheight) caption:"$caption" \
		"$file" +swap -gravity south -composite "$file" && \
		printf "\n$file watermarked successfully\n"
	fi
	shift
done