making killer thumbnails goes down to 5 steps
STEP 1 : Make sure that the video idea is on somthing super interesting the title and the thumbnail must work toghter because the title and the thumbnail represant the idea and if the idea isnt interesting enough no one is going to click on it
STEP 2 : getting the potential viewers attention sucssefull youtubers applying the 2 steps formula
1 using contrasting and complementary colours in their thumbnails
2 provoke an emotion in their thumbnails such as fear surprise curiosity