I wrote everything below this paragraph first, and then I realized that you only want thumbnails. You can enable monetization and just don't monetize any of your videos. You should still get custom thumbnails. Feel free to read the paragraphs below if you want more information about monetizing gaming videos.
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You can enable AdSense. Most people will tell you not to since you're a gaming channel, but I also have a gaming channel and have had a lot of success with AdSense. As long as you clearly state that you can monetize the video through Fair Use (and that means you need to quote Fair Use, not just say you can use it), then YouTube will allow you to monetize the videos.
Minecraft is an exception. If you look at the terms on their site, it says you can do anything with videos made from the game. That means you should quote Minecraft's terms instead of using Fair Use.
The only issue I've had with AdSense is that for the past 2 months, I haven't been able to monetize a single video, not even Minecraft. It's probably YouTube's automatic copyright detector that's causing an issue for me, but I have no idea how to fix it. That's something to consider.