The quickest way to get a cover for your book is to get a big house (or any other house) to publish it. All they want is your book text in some sensible Word format, and from that they will create the interior design and the book cover. Oh yes, your suggestions about what that book should look like will fall on (their) profoundly deaf ears, they won’t pay you much or often, and they will also change your titile.
To get your own cover (and titles), self-publish and/or work through ancillary publishers. But then you are responsible for the cover artwork. If we publish through Lulu, CreateSpace, or Blurb, we can create acceptable covers through each, though they will differ from each other.
If the book is digital–for Kindle, iPad, Smashwords, Scribd, and LightningSource–there are two schools, though you need some cover image for all. One says that the covers are almost immaterial since they are seldom printed beyond a wee thumbnail. The other school, mostly novelists and perhaps poets, say that the cover must look sharp and professional. Take your pick.
So most of us will probably have a cover designer create a cover that can be adjusted a bit from the bound book to the digital version, to which an ISBN and the appropriate price can be added, if needed. That way the same book can be published by many houses and it will have the same external appearance.
Yet to do that you now need many different cover files from your designer.
Not too many years back, I only needed two kinds of covers from my cover designer: a full cover .jpg (front, back, and spine) and a front cover (also .jpg) for the digital version. When it came to promo materials, mostly fliers and cover downloads, I also requested .pdf copies of the both covers. These were in addition to the designer sending the approved cover artwork to the printer. He called his counterpart, I presume, at the printing firm so that when I sent my book text as an email attachment (in .pdf format), he also sent the cover(s) the same day so that both were linked to the same proof!
Lately, with ancillary publishers added to the submission mix, we also need cover files that are 600 pixels or larger (up to 1,000 pixels is fine) so the covers can also be used (and easily read) in thumbnail formats.
That’s the core of the work-for-hire agreement I have with a cover designer: possible designs for the cover, my selection, my final artwork OK, its submission to the primary bound book printer (I will submit it to the ancillary houses), and the additional files mentioned above that I keep on disc and at the book file on my computer. That way, with the publishing choices at hand, I can paste and submit as needed without becoming a pest.
Some add postcard and bookmarker artwork in the same package. A friend also has poster art included. Ask cover designers you are considering if they have any other cover file ideas they can add to the package.
P.S. My newest book tells you how to publish through those seven ancillary publishers, and addresses the covers in each case. The book is called How to Get Your Book Published Free in Minutes and Marketed Worldwide in Days? I also have a free monthly newsletter where I mostly talk about publishing and writing, plus empire building. Glad to have you aboard!