In my experience, there are 3 things that every good thumbnail/ channel art has: 1. Simplicity Thumbnails that have lots of words and colors is a major turn off. Having a simple background color (usually black or white) along with a picture and one word, that alone can easily attract viewers. For banners same thing. Do not have a complex banner other wise no one will know what your channel is supposed to be about. Again have a simple background with a picture and your channel name. That is it! Sounds simple but it will help viewers find you better! 2. Quality Blurry pictures equals no clicks today! It is hard for a viewer to choose the video if they can not see what it will be about. Words mean nothing these days, viewers look for pictures only! The better looking thumbnails are usually the videos with lots of views! So make sure your thumbnails are in top shape! For channel art be creative! Express yourself and not someone else! That is how people will get to know you better! Keep it real and keep it at a good quality. 3. Expressive Pewdiepies thumbnails are very good because they alone can tell a viewers over a million things. Thumbnails that can tell you a story with just one glance at it is a big seller. It is like writing a book, so put the necessary details that will portray your content exactly! For channel art, put stuff that is related to YOU not the videos! Your not some market place your a person! Show that you are one! Express yourself in however mannor you want, people like that! So with these 3 things in mind now your thumbnails and channel art will be perfect! Hope this helps and have a nice day! Peace!