Thumbnails are an interesting part of graphics design for channels, and some people take it a lot more seriously than others. Personally I don't try too hard on thumbnails, I simply google the name of my game, and browse the images for a good background or screenshot that does a good job of showing what that game is all about. If it has the game logo on it, all the better! If it doesn't have the logo, I just google Game Name Logo PNG. Most png logos will have "clear" backgrounds and can be dropped onto your other image, in any image editor, and it won't have a square white or black background. After that I just hit up Dafont for a font that fits the game, whether it's horror, action, nerdy, fancy, etc. and type whatever text I want on the image. This whole process takes me about 3 minutes to do nowadays, but some people take it way more seriously and spend 30 minutes for every thumbnail they upload.
