A simple cover design consist of a single image without any added elements, not counting your text. There are designers who call this "slap text" designing because most people will slap text on an image and move on. 

You can create beautiful single image covers if you devote your time to it. Just because it's simple doesn't mean you can't do adjustments. Like color balance, brightness/contrast, and so on. 

The is the base image for the cover -

Here is the cover I made -

The image chosen was dark and also black and white. It lent itself to a thriller or dystopian feel for the cover. I added two adjustment layers, I adjusted the exposure and I added a red layer set to soft light and at 60% to add a little color. The font is a slightly eroded serif font.

Just because it's a single image cover doesn't mean you can't create a stunning end result. As with everything, you simply need to take the time to do it and practice.