When you need a cover for your new book, you have several options:
1) Create the cover yourself, which is recommended against if you lack designer skills.
4) Hire a professional designer. See: What to expect from your book cover designer.
How can you tell whether a cover design is optimal or not? Here are three critique and feedback resources:
Monthly e-Book Cover Design Awards by veteran publisher Joel Friedlander, with helpful comments. Below is the November 2016 winner, created by Sig Evensen (Inkubus Design). The judgment is that the cover “captures the hardboiled, pitch-black gallows humor theme of the book perfectly.”
Reedsy regularly runs live Cover Critique sessions in which designers have their say about covers. Submit your own cover to get feedback from a pro. In the video session with designer Jake Clark, he comments on the cover below: “Instantly comes across as creepy. The choice of image is good and makes you feel uneasy. It avoids the trope of excessive gore in horror which is good. The text can be read easily, but the author name can be a little bigger.”
Derek Murphy is a seasoned cover designer. “Sometimes excellent books need a facelift,” he says, to give them a more professional look. Here’s a before-and-after example: