Using the above example of a horror acim you could have a scene from the book where the main character is trapped by the main monster or villain. You would then refer to the chapter in the book to note small details of the surroundings you may have mentioned. You may have the main character trapped in a cellar with their back up against an old wooden door as part of your book.
This could be used as a perfect book cover design idea because you will give the reader a visual account of what your character looks like and also an image of an important scene.
Referring to the example about the success of social media I mentioned above you could have a picture of a group of happy looking business people celebrating their latest social media campaign.
The scenes available to you are only limited by your chosen topic, book content, and imagination. Your next choice is the technique by which you intend to actually bring your book cover design ideas into reality.
You can of course hire someone to create a cover for you, which is not as expensive as you may think.
There are plenty of budding artists and photographers that are trying to establish a reputation. Most of them will charge a fairly low price in return for the publicity your book will bring to them via a book cover design credit that lists them as the creator.
If you have followed the steps above and have a good idea of what you are trying to achieve in your book cover then you will be able to give the artist a framework to refer to.