Ironically spent the last two hours researching the topic nothing out there bar one or two ways to publish, but this is exactly spot on what's missing it would be massive, provide a store to view them for authors creating them, maybe even share them on their behalf if checked before clicking publish, so that they appear on amazon etc etc but maintain anonymity.
But absolutely all the tools to manipulate imagery and add it your self, and maybe also in the future if the site is made which I absolutely know it should be, it will be hugely popular definately, then you should add some paid art and public domain art to add royalty free no attribution required 100% legal to sell commercially so we don't have to source it outside the site a bit like canva for books with a store of your own to share promote and sell in and maybe do the farming out to other online stores on behalf of customers to other retails outlets online.
Got a suggestion on a name alternatively to widbook how about "covered" or something pertaining to a part of a book like "coverdapp"maybe cooler
And maybe add what lot's of other picture editors online fail to which is lens flare of multiple type and after effects. :)
RE: Widbook - Platform for writers. Explore the world of digital books...!