How Auto use content post image as thumbnail

Hello.

I just moved to a new Theme and i have little probléme with the
Video (Youtube) category that doesn’t contain Featured Thumbnails,

There is images inside the posts so i wanna know if there is anyway
or plugins that will scan the specified category and use the
contained image inside post as a thumbnail

This plugin do the Job : Auto Post Thumbnail

But only probléme is that he doesn’t filter by caterory so it missed
up other posts.

Anyone can Light me pls !