Mind you that I'm not an artist, so take what you're seeing with a grain of salt.
The idea here is to showcaase the expansive "evolution" tree that you have in the game. You can use your assets, they work well!
Try to show some more interesting materials, even if it's not 100% gameplay accurate. With the thumnail, you're selling the idea of what experience a player might have with the game, not how it actually works.
In your case the cool aspect of the game is "I can go from funny stone to god awful abomination", or something like that!
Of course try to also not spoil the cooler transformations, but you'll be the judge.
Hope this helps <3