I am an occasional PC Gamer and only visit Gaming YT channels once in a while. I stopped by your channel to see your thumbnails.
I am from the school of thought that if you are going to use text in a thumbnail, it needs to be large enough to be readable! Personally, I do like a little bit of text to help describe the video (but don't get wordy).
I also tend to like thumbnails that pop out with high contrast colors. If you are using an image that is kind of bland in colors, you can make your thumbnail pop by using a "loud" color like Bright Red, Bright Green, Bright Yellow, etc. on a darker background.
I am a big believer in visual branding. That means you should try to work towards a reasonably consistent style. Your thumbnails vary a lot in style. So it gives the impression you are sporadic in your channel.
I am with everyone else, your Zombie thumbnail is clearly the best one. Keep studying and watching other people's thumbnails. Over time, you will evolve to your own thumbnail style.