For covers on the paperwhite, you have to either convert the file to .azw or convert/reconvert it to the same format and remove the [PDOC] tag and make it an [EBOK] tag
Otherwise it shows up as a personal document instead of an ebook. I just convert them to .azw though since its easier. Seems like azw files are slightly larger though. azw aren't compatible with all devices though, i'm not sure if they work on the android app or not
Also sometimes reconverting can change the formatting of the book. Like it'll introduce spaces between paragraphs, which is really annoying to me. Another reason to just convert everything to azw and keep your mobis or whatever around for other devices
Dunno if you can change the personal document tag on a pdf or something without converting it.