I like to use Croppy since I post on Tapas AND Webtoons.
https://knicknic.github.io/croppy/
A big part of posting webcomcs on sites like Tapas and Webtoons is that you can upload multiple files, so if your single image is coming out low res then this is a good solution. Export at a larger size, then put it through croppy-- which will do its best to give you multiple images (named in order with numbers) that you can upload instead.
Of course this also depends what size you are originally working in.