It takes some time, but testing alternate thumbnails long after you publish your videos is worth it.
I took a quick screenshot of a recent test we did and TubeBuddy provides some interesting insights.
For example: Even if your your thumbnail get fewer views, that might not matter if you're getting the right person to clickโand your watch time goes up.
You can see the details in the attached screenshot.
We've seen a major uptick in views since Hormozi announced Skool so we've been adding the Skool logo to most of our Skool related videos (and its working).