How to add an image to each blog post in the blog post page/list

I guess this is pretty trivial – but haven’t found the answer though… I am developing a new wordpress blog, with the theme Savona in the moment. When displaying the page with the blog posts, I would like to see the image I include to each post displayed as well. It’s not the “featured image”, in principal, although that could be a workaround. I have done that years ago with my principal blog: https://lebensraum-permakultur.de. But don’t know how to achieve that with the new theme. Any suggestions? Thanks for an hints!

The page I need help with: [log in to see the link]