There are some general rules you should follow when creating a thumbnail.

• A “strong“ picture that tells a story
  
  1. If possible a person's face with a big and obvious expression.
  Laughing, sad, happy, etc..
  The white of the eyes should be visible.
  People feel a connection to these kind of thumbnails and click on them more often than on other thumbnails.
  
  2. Something cute that touches the hearts of your visitors.
  
  4. Yummy looking foods are great, too.
  
  This all depends on your niche, of course!

• Text in your thumbnails
  
  This is especially important for Youtube, but can be used on your website, too.
  
  1. Max 3-4 words
  
  2. If on Youtube the bottom right corner has to be left free!
  In this corner is always the length of the video displayed and you don't want your text be covered by that.
  
  3. The text should include at least parts of your keyword and be in harmony with the image.

• Bright colors and contrasts
  
  While not blinding your thumbnails should be clear and colorful.
  People like visuals and when your thumbnails are pretty they are more likely to click on it.

Your thumbnail really has to represent your content. It has to catch the attention of your visitors before they move on to another site.

You can't put a sexy girl in the thumbnail, if there is not a sexy girl in your video/content. ;)

Have a look at the channels of famous Youtubers. They all do an exceptional good job with their thumbnails.

(look up "Tyler Oakley" on Youtube to see fantastic thumbnails)