Customizing video size format and blurry Thumbnail

Site URL: https://www.leilfornar.com/vlog

Password: Aloha

Hey Guys!

I am using the video page feature for a project; instead of uploading videos to the series, I used a Vimeo embedded link. I pasted the website and password.

However, now the Thumbnail proportion is a) not 16:9 and b) super blurry. Weirdly, on this page feature uploading a custom thumbnail is only available when I upload the video to Squarespace but not when using the Vimeo link. 

Does anybody know how to fix the bad Thumbnail quality and change the format of Thumbnail?

Thanks, Julius