How to show thumbnail image from YouTube in home/archives/etc

Hi, I tryed to make what is discussed in this post:

[resolved] Show images & videos on home page

This is the result – Amazing!

[off-topic: the 3th video don’t show, why? I put him in a video post, in the same way as the others. Is a youtube problem? Maybe this video url don’t support “embed”?)

I really like, but I think this could be a problem for those who have poor conection, or poor hardware, it’s correct?

So, I want to show the Youtube thumbnail image. It’s possible to do it automatically? Right now, I’m making my own thumbs images, and put in Featured Image. But I will have at least 350 post like this. It’s a lot of work make any single thumb.

Thanks