Misconception 1: The Author Has Full Control Of The Cover Art
Ha ha ha! No. Not even close.
In a big publishing house the cover art is chosen by the marketing team. They look at other books in your genre and price point and find something that fits. Whether it actually fits the book is a secondary concern. If you are lucky you'll get to send in a few ideas. If you aren't, well, there's a reason there's an apple on the cover of Twilight and not a vampire.
With a smaller press the artwork will be handed to the designated cover artist, who has never read your book, and they will do their best with the description you've sent them. If you don't like their interpretation you might just have to suck it up. If you're lucky they'll change it, if not... too bad!
Misconception 2: You Will Have Original Art For The Cover
Oh, honey, you're adorable. It's not happening.
The bottom line is king in publishing. Unless you earned a substantial advance there is very little money set aside for marketing, and most of it will go to paying the publicist, cover artist, and anyone else on the marketing team. Big press or small, if you aren't a Big Name you are getting stock images.
Publishers save their full court press for the books they've invested money in. That's just life.
Misconception 3: Your Cover Will Never Change
o.0 Have you never heard of Harry Potter?
Covers change based on the market. If you have a bestseller you may see several (if not hundreds) of evolutions of the cover over the years your book is on the shelf. If your book sells internationally there will be different covers for the different regional markets. The USA cover isn't the same as the UK cover isn't the same as the Brazilian cover.
Misconception 4: The Cover Will Be Beautiful
You sweet summer child.
The cover will be marketable. What "marketable" means varies by press. Whether the author will love the aesthetic and design is another matter. Whether the readers will like the cover is completely beyond your control. Accept that at least one person will give you a 1-star review because they hate your cover and move on with your life. Some people will never be happy.
What Can You Do About It?
Roll with the punches.
The better your sales the more clout you'll have when it comes to covers. Maybe. To a degree. If you absolutely must have control over your covers than consider self-publishing or working as your own cover artist. Smaller presses may consider letting you make your own cover if your skills meet their standards. For larger houses, let the marketing team do their job and focus on writing.