I'll say what I always say to everyone asking similar questions. What makes you click on a video when looking at the thumbnail? Is it the image in the background? The text on the thumbnail? The boxes and circles used in the design of the thumbnail? Depending on the content you are making (gaming, vlogs, how to's, animations, etc) the thumbnails will have to capture the viewer's attention just like they do to you. Use what works on you as a template to create thumbnails and change them until you find the right combination of things to include on your thumbnails. But above all else make sure the thumbnail tells the viewer what the video is about. Otherwise it will be a waste of time. Also, don't clutter the thumbnail too much.