Kind of a simple thumbnail or icon, there is just a thing that I read, that made me think:
NOTE - For every version update for the game there will be a new thumbnail added.
If you mean that you will use the same thumbnail only changing the number, I think it will be a lazy work, and perhaps not the best, but maybe you could use it as a template, but instead of showing a big image of the “Sonic plus+” logo, you could show what the update brings + the version and a small logo in the corners.
It would both, make more sense and make the thumbnails tell you more of the game. This is just my opinnion, but for the images you showed, as a basic concept or starter concept they seem fine.
Also something that made me think about it, the game is related to sonic, so it should be made without any possible thing that is paid with robux, gaining robux from copying or using charaters could get the game in problems, so just as a small tip, be careful with it.