Is it to increase click rate? I wish I knew myself haha.
General things I think that can help improve your thumbnails. But take it with a grain of salt.
1. Using more stylized font or custom font (i.e. Final Boss fight was a good use of it)
2. Ensuring font doesn't blend into the rest of the image, making it hard to read (i.e. high command and goblin raid)
--> This could be due to the colors being close in range, or there is too much noise near the font.
Let me know if you learn some other tips and cheers~