Changing Book Cover for Kindle for PC app

[saranicholle]

I don't actually have a kindle or anything, so I use the free Kindle for PC app.
I want to change the cover art for some of my ebooks, either because they were free Gutenberg downloads and have no cover (very confusing to find anything), or there are just cooler versions of the covers that I would rather have.
Using Calibre, I was able to get the covers how I wanted them, but I can't figure out how to transfer the new cover to my Kindle app. I tried deleting the first version of the ebook, saving the Calibre version file, and opening the Calibre version with Kindle, but the covers remained the same/blank (however, I did change some of the book titles - for instance, adding "Book 1" "Book 2" etc. for series - and that changed just fine).
Any ideas on how to get this to work with Calibre? Or is there an altogether different program/method I need to use to accomplish my goals?

Also, totally random question I know, but: is there a way to get the kindle app to group books of the same series together in order?

Thanks in advance for your help!

[speakingtohe]

You have to convert the book mobi-mobi to embed the cover.

Helen