how to grab first image in a post automatically

Hi,

I would like to grab the first image from a post and show it. I tried several plugins but nothing works. I also added code in my fonction.php and loop theme, but did work. I have never used featured images for my old articles. i have only the first image into my old posts.

Do you have a solution ?

My theme is Sight : https://wpshower.com/themes/sight/

Thanks,
Martin