Actually, it didn't used to be like this?
Croppy had an extension that could easily slice/crop/resize everything into Webtoons and Tapas. I believe that extension still works in Tapas, but Webtoons changed their site code. Hence, the extension no longer works with Webtoons. So if Webtoon is your main site for your comic, blame them.
Moreover -- it's as simple as running your cropped files through tiny.png or tiny.jpeg to make them smaller if you worked in a size that's bigger than the norm. That, and you can always delete the extra slices once you've posted on the sites and have kept your needed backup.
Compared to trying to buy from Photoshop or trying to manually do it yourself, as well as trying to do it your way but waste more time -- Croppy is your best bet for now.
