One suggestion is try to make your thumbnail original. It is easy to go to google, find an image and slap on some text. But the thumbnails that really sell are the ones where people take their time to make each aspect of the thumbnail stick out. (If the DBZ image is your original work, then I apologize. It looks good but you need to work on the text). Like some replies before me said, the white font is not sticking out. The simple solution to this is use word art and have a black border/outline on the letters and use white fill. This will lead to your text standing out.
If you want a great site to use, try
canva.com. It is completely free and has an easy to use interface. If you decide to use it and need help, send me a private message and Ill try to help you the best I can.
Three tips I have are:
1) Make your purpose/title stand out from the rest of the background.
2) Use multiple images. Pick one background image then add some smaller ones in.
3) Increase saturation. Mess around with the filters.
Beside those things, the video looked great! Fun and interesting editing