This is exactly what I've been trying to do with https://thumbnailmaker.art. Source: about 1 year ago

The users come from Thumbnail Game, and they vote which thumbnail they think is better. You can post your thumbnails and even choose other thumbnails to compare yours to. I'm hoping this will be useful for knowing the most effective thumbnail before you post on youtube. Source: about 1 year ago

I made a game where you guess which video has more views given just thumbnail and title. There are over 200,000 users. Source: about 1 year ago

We built a tool where you can upload your thumbnails and titles and they will be directly shown to users playing our web game. Our players will vote on your thumbnails so you can see which ones are the most clickable before you post to YouTube, no more switching out your thumbnails required. It's free to use, and we would love your honest feedback. Try it out here: studio.thumbnailgame.com. Source: about 1 year ago

There's actually a cool site that let's you test our your thumbnails before you post and get hundreds of votes on them and they have an AI YouTube title generator. It's called Thumbnail Studio and they serve your thumbnails directly to users playing their web game . I'm pretty sure they are in beta so I was able to use their tool for free without credit card which is nice. Source: about 1 year ago

I own a game called Thumbnail Game where you guess which video has more views given only the thumbnail and title. I added a creator tool where you can upload different versions of thumbnails and titles and they will get pushed out to the game where players will pick the "better" thumbnail. It could really help to gauge what thumbnails people like more before you publish your video on YouTube. Source: about 1 year ago