1. But if I have an epub file with absolutelly no image in its structure, is there any way of telling calibre not to create any kind of cover? No default cover, no first page screenshot - nada. The "cover page" might be the first HTML page, but I don't want any JPG to be created like the first page screenshot. Can I ask this to Calibre somehow?
2. Ok i realize that if an epub has an image inside it's structure (which is not the cover, for example is the publisher logo, or Tolkien's Middle Earth map) but was dumbly named cover.jpg (buy the person/software who generated the epub), and I add a new cover, the cover.jpg file will be lost upon export to device - so the only solution is to FIRST edit the epub file in sigil (or manually) and rename the original cover.jpg in -let's say- img0.jpg, BEFORE adding the new cover.