A script that adds to the images files stored at "~/.cache/wallpaper" a mark containing the display ID and the resolution, both extracted from the filename. After executing the script, restart the session and then the mark appears, as the cached files are loaded.

test_wallpaper_sizes.sh

#!/usr/bin/env bash

if ( ! command -v convert >/dev/null 2>&1 ); then
  echo -e "\"Convert\" is required but it's not installed."
  echo -e "Install \"imagemagick\" package and retry."
  exit 1
fi

case $XDG_CURRENT_DESKTOP in
  "X-Cinnamon") DIR_CACHE=~/.cache/wallpaper ;;
  "MATE") DIR_CACHE=~/.cache/mate/background ;;
  *) DIR_CACHE=~/.cache/wallpaper ;;
esac

for f in $(find ${DIR_CACHE} -name "[0-9]_*"); do
  disp_id=$(basename $f | cut -d "_" -f1,3,4)
  convert -pointsize 50 -fill yellow -draw "text 50,100 \"${disp_id}\"" $f $f
done                                                                           
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