I'm very glad you commented, because I have a lot of information for you that will help you do even better than you already are.
Firstly, I just took a look at your page. Your reviews look really good and I like your use of custom designs and you have good taste in movies. Using a high quality image of the movie poster is an excellent idea.
The way that I'd change your thumbnails to make them more personalized is by turning those vertical posters into landscape rectangular images. You can do that by adding a background, maybe other images, and text. I've linked some apps at the top of this post if you don't already have some.
I'll also send you many examples of how I used to do my movie review thumbnails. I've written many over on CineTV. I even have a collection with all of my movie posts in an easy to find place. Here is the link https://peakd.com/ccc/daltono/movies-and-tv-shows
Let me know if you have further questions. I think once you look at my movie thumbnails, it will give you a great idea where to go with your own.