A few things to remember with your thumbnails, is to make them easy to see. Some nice colors, a readable font when the thumbnail is small, and some sort of logo on it help a lot!
Here are some examples.
Game Grumps Spoonymass Toby Games
All of these are eye catching. In fact you seen them before you even started to read. That is how well made they are.
Another thing you may have not noticed was, your eye was attracted to the right one immediately. A bright green can catch the eye very quickly. You can see this all over Youtube right now because of a channel called Game Theory.
Green has been proven to work wonders on the Game Theory channel. I have also found some light blues work well at times, but there is another thumbnail trick that works great.
Markiplier Jacksepticeye PewDiePie
Your eye might have been attracted to these thumbnails before the green font ones. That is because our brain looks at faces to recognize someone right away. It does not matter who it is, or if its a cartoon/game character. Our brain wants to know who it is.
This does not mean fill the interwebs with bright green faces! It just means that you now know how the brain works when it comes to a thumbnail.
Got any thumbnail tips? Pop them in the comments below and share! I will update this with your useful tips!