Featured image to run full width under header (blurry)

Hello,

I would like to have my pages’ featured image run full width uner the header.

I have inserted this css code:
img.header-image, img.wp-post-image {
width: 100%;
height: auto;
}

But my image comes out blurry, it seems it uses the wrong size of the image.

How can I fix this?

Thank you.
Zsuzsa

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