The message of your cover should be felt by a simple glance. A client at times cannot spend too much time trying to see and figure out every detail of your cover art if it's all cluttered it up. You can get the best ideas for Book design services through various online sources.
The title, author's pen name, and any words written on it should be readable. Try not using too many colors, your book cover is not a rainbow. Harmonize everything perfectly so that it can look both striking and enthralling.
Showing a glimpse of what your book is about: A picture speaks a thousand words. Who hasn't heard that saying? And we all know it's true. To ensure your cover can speak for itself with the imagery you provide it.
Projecting the core of your story Some use two or three words, others use a short sentence. It's not easy, but as the author, you need to take time to ponder. How would you describe your story in a few words? Once you nail down a few catching phrases, then those are the words you want to add to your cover. Practice makes perfect. Again, rummage through other book covers and see what phrases they use.
How can I avoid any legal troubles?
Always be careful when using photos or illustrations that are copyright material. Only use those that you have been given written form permission to utilize, or those you purchased rights to use. You can find many pictures to purchase rights to places like a big stock photo, shutter shock, 123rf and etc. May you have success in making your own book cover.
Although a book cover is essential in making a load of sales, we cannot ignore the fact that quality is the key.