Hmmm I have to look into this more in depth. However, that's a great question! For me, since I use Draft2Digital to distribute all my ebooks to major publications I simply upload the pdf ebook cover file I created. From there "I think" it merges it together to create the ebook. I could be wrong, but I love draft2digital as it takes care of everything. It's "free" in the sense that they take a small cut of your royalties when you make a sale BUT it doesn't cost you anything until you make a sale. I know other platforms charge monthly, but with this platform they win when you win. I hope this may offer some help!