To create thumbnails directly into Synology NAS (if you want to make thumbs faster). From https://github.com/voz/synothumbs/blob/master/synoThumbs.sh

synologythumbsphoto.sh

#!/bin/sh
# Author:       phillips321 contact through phillips321.co.uk
# License:      CC BY-SA 3.0
# Use:
# Released:     www.phillips321.co.uk
    version=1.0
# ChangeLog:
#       v1.0 - now supports video conversion (mov and avi)
#               - for more formats please contact me with requests as i dont have any sample mpg etc
#               - ffmpeg output is surpressed (>/dev/null)
#       v0.1 - first release

XLname="SYNOPHOTO:THUMB_XL.jpg" ; XLsize="1280x1280";
Lname="SYNOPHOTO:THUMB_L.jpg" ; Lsize="800x800";
Bname="SYNOPHOTO:THUMB_B.jpg" ; Bsize="640x640";
Mname="SYNOPHOTO:THUMB_M.jpg" ; Msize="320x320";
Sname="SYNOPHOTO:THUMB_S.jpg" ; Ssize="160x160";

#help message
if [[ $# == 0 ]] ; then echo; echo " Error: What directory to process?"; echo " Usage: $0 ."; echo; exit 1; fi

#main block for pics######################################################
for i in `find ${1} \( -type f -a \( -name "*.JPG" -o -name "*.jpg" -o -name "*.png" -o -name "*.PNG" -o -name "*.jpeg" -o -name "*.JPEG" \) ! -path "*@eaDir*" \)`
do
  picName=`echo "${i}" |  awk -F\/ '{print $NF}'`
  picDir=`echo "${i}" | sed s/"${picName}"//g | sed s/.$//`
  echo "Searching Thumbs For $i"
  (mkdir -p "$picDir""/@eaDir/""$picName"; chmod 775 "$picDir"/"@eaDir"/"$picName";)
  (convert -size $XLsize "$picDir""/""$picName" -resize $XLsize -auto-orient -flatten "$picDir"/"@eaDir"/"$picName"/"$XLname"; echo "   -- "$XLname" thumbnail created";)
  (convert -size $XLsize "$picDir""/@eaDir/""$picName""/""$XLname" -auto-orient -resize $Lsize "$picDir""/@eaDir/""$picName""/""$Lname"; echo "   -- "$Lname" thumbnail created";)
  (convert -size $Lsize "$picDir""/@eaDir/""$picName""/""$Lname" -auto-orient -resize $Bsize "$picDir""/@eaDir/""$picName""/""$Bname"; echo "   -- "$Bname" thumbnail created";)
  (convert -size $Bsize "$picDir""/@eaDir/""$picName""/""$Bname" -auto-orient -resize $Msize "$picDir""/@eaDir/""$picName""/""$Mname"; echo "   -- "$Mname" thumbnail created";)
  (convert -size $Msize "$picDir""/@eaDir/""$picName""/""$Mname" -auto-orient -quality 60 -resize $Ssize "$picDir""/@eaDir/""$picName""/""$Sname"; echo "   -- "$Sname" thumbnail created";)
done


echo "Done"
echo ""