Thumbnails for Index page !! Help please

Hello,

I want to show thumbnails on my index page (mean recent posts page : https://www.chaibialaa.com), and, I would like those tumbnails to be generated from uploaded picturs or from post pictures and that they have a little birder grey (design but not a problem). Also, the most important thing is that thumbnails are shown on the Index page and NOT into the post first place !
On this theme this is a great sample of what I need (Thumbnails generate picture from posts !https://demo.jauhari.net/?wptheme=Hamasaki)
My site is chaibialaa.com