How to upload a big webtoon image without taking away image quality?

I recently started a webtoon canvas series, but when I went to upload a couple of pages onto a chapter, it turned out they were way too big, so webtoon did it's thing where it sliced and compressed the images, but now the image quality sucks. In the future, I'll use smaller image sizes, but I really don't want to have to redraw the first couple of pages. I tried using this thing called Croppy, but it the image quality was still really bad, even when the pages were split up. It's nothing wrong with the site, I think, it might be because it doesn't crop images horizontally, just vertically. Is there any way to get the images slices without taking away image quality?

I'm really bad with words, I'm sorry, so feel free to ask any clarifying questions.