thumbflip.co is more customized than YouTube's "Test and Compare" feature.

1. Upload up to 10 thumbnails per video
2. Choose number & frequency of rotation - hourly/daily/weekly
3. Choose your own test duration
4. Metrics per thumbnail available on the next day after the test ends - views, Avg. View Duration, Likes, Comments, Impressions, Click Through Rate, Shares, Subscribers Gained, Average View Percentage