The first thing your reader will see is the book cover, You can just download pictures off the internet? No. Many images are not copyright free. You cannot use them without permission.
You can search for copyright free images, but they tend to be generic, and you want your cover to stand out. There is no easy answer I’m afraid. I had help with mine. I subscribed to Canva, but found the whole process overwhelming. My attempt was more like a child’s collage at nursery.
I got invaluable help from my daughter, and was pleased with the result.
If you self publish with Lulu, or Kindle direct, they have their own stockpile of images you can use. Just make sure that the cover fits your story. Get the viewpoint of other people. Ask them if they like it, or dislike it before committing.
Just like editing, there are designers out there that will create your cover. Make sure you get several quotes before spending your money. It can be expensive. If you do decide this route – do your research.