they're not that bad
if you want i suggest following some photoshop tutorials

Practice always makes perfect. It took me over four-five years before I felt happy with my thumbnails. What I learned was to always save Photoshop photos in .PNG and never .JPG as .JPG lowers your quality every time you save as .PNG never lowers your quality. With any type of Minecraft thumbnails I will usually go into Mine-imator which is a animating software, 100% free. I will pose my characters, environment and then add lighting to capture the best photo possible. Then I add that photo to photoshop, add shadows, add some color correction, add text if its needed. You really just need to fool around with settings until you find your style. Mine-imator is just really handy as it has all vanilla items, blocks and you can post everything anyway you want!

Have a great day! :)

Not that mines great, but I suggest adding widgets with images or text that relate to whatever aesthetic/theme you're going for.