The current wisdom in marketing books is that the art should give the "flavor" of the book. Check out these Harry Potter Book Covers on google images. Or here is the 15th Anniversary edition of Harry Potter and the Sorcerer's Stone:
This is the kind of cover you want, notice Hagrid, Harry, the Owls, the cobblestones and a cast of fantasy characters and buildings. Of course this doesn't have to be a scene in the novel (I don't think it is), and this is what is meant by "flavor"; this is all fantasy. In general, you DO want characters on the cover, nothing attracts human attention more than a human face or eyes.
For artwork, I'd recommend (I have no involvement and get no commission) www.fiverr.com, they are relatively inexpensive, you can see the type of work they do before you hire them, they will interact (you can get line sketches, revisions, etc) and I have used more than one artist there that could have completed the cover above. That said, buyer beware, as always. They are independent contractors, often out of the country. Most offer a few revisions, but if you still don't like what you've got after a few revisions, the site runners tend to give the artist the benefit of the doubt and pay them. Don't expect a refund.
For that reason I suggest you commission in parts, this is better for the artist and for you, they get paid for milestones, and you have the rights to what they do, and can drop an artist after tens-of-dollars instead hundreds of dollars.
I have begun with getting rough line-sketches of key characters for a small amount ($10 each) based on my descriptions. Then a line sketch of a composed scene. If I am still happy, yet another gig to finalize that and color it in. The more complex the art, the more expensive it will be.
I suggest you google professionally published best selling books in your genre (not self-published), and look at the components they include to set the scene. Don't fall into the trap of thinking you need to illustrate a scene from the book, many illustrations show the cast of main characters (two to five people) with one main character dominant. (Of course there is flexibility, the front-and-center character in the cover above is Hagrid, not Harry).
I'd strongly recommend against abstract art or anything without a character in it. Some beautiful abstract art (like colored fractals) can be eye-catching, and sell something, but in the end stories are about characters and we are evolutionarily wired to pay attention to the images of humans and animals (the unpredictable elements of a scene IRL), more so than we are to beautiful nature scenes or abstract art.
You want your scene to evoke some sense of what an immersed reader of your book will feel; it is what is inside the book. Like the Harry Potter cover above; there is wonder and medieval magic inside this book, giants and witches and strange creatures in a candle-lit town. And it is kid-centered.
Romance covers have two youngish (unwrinkled adult) lovers on the front, often with the flavor of their world behind them. That is what you will find inside.
Scifi covers often have a character, space-suited perhaps, holding or wearing gadgets (light sabers!), or may depict a space battle, or some strange alien artifact on a world, etc. In general, they are showing what you will find inside.
Detective or mysteries often show a detective detecting, perhaps with a victim. Sometimes in danger, or surreptitiously observing something. That's what is inside.
Now, major names can get away with just their name and the title writ large over some simple art. Don't be fooled by that! Their name alone sells books to their fans. If you don't have a fan base that recognizes your name and will buy anything you write, keep it small, The title can be large but not overwhelming, (like the Harry Potter cover above), and let the artwork show potential readers an image of what they will find inside, throughout the book.