I have CSP but I don't slice the image up in CSP, I use a program that auto slices the picture for me. Its called Imagemagick. You'd need to install this and then here is the command I use:
magick convert .\filename.png -crop 800x1280 -quality 100% -scene 0 .\foldername\out-%d.png
Use this one to keep a file in png format - cutting images to size 800(wide) by 1280(long)- you can adjust those numbers. -scene 0 is for stating what number I want to label it. So like right now it will do "out-0.png" for the first file then "out-1.jpg" for the 2nd, etc. But if you change scene 0 to scene 15 it will start at 15 with the first file being "out-15.png". I sometimes have really long comics, and you can't make png pictures longer than 30000 pixel length. So I have to run multiple pictures through the split and I want the file names to have a an order.
magick convert .\filename.png -crop 800x1280 -quality 100% -scene 0 .\foldername\out-%d.jpg
to auto convert the PNG file to a jpg for LINE Webtoon.
It's super fast too.