Where to place main image?

Hi,

I had a quick question. I am using twentyfourteen theme and have a small problem placing main image.

here is the website: https://cooking-the-world.com/

Right now my image is placed at the bottom of header.php, but as you can see, this messed my sidebar. When i place it at the beginning of content.php it appears on every blogpost i write.

Do you have any ideas where I could place this main, big image, so it appears next to the sidebar and under the header?