I love Fany's tutorials! Art and information is fantastic! Before reading her tut on the slice tool I'd actually use the crop tool in photoshop and manually divide the image up that way. Oh it's so much easier with the slice tool. lol
It's 4000 pixel height restriction per image you upload into an episode and episodes can contain like 20 to 30 images(?)... a lot, it can contain a lot of images. So basically for your 15,000 pixel image you'd use the slice tool [as shown how to do it in Fany's tut linked above] and divide your image by 4 horizontally. (since 15,000 divided by 4,000 is 3.75 you'll need to round it up to 4 slices to make it work)
Then "Save For Web" in photoshop and it'll save all 4 slices as their own image files. Next upload those 4 image files into a Tapastic episode and BOOM! Done!
Webtoons has a smaller image file restriction than Tapastic. It's 800 px wide by 1280 px tall and they only take jpg files, so you'd run into the same issue there. You just have to slice your image.