deformation header image

Hello, I tried to add an header image. On a laptop screen all is well; on a mobile scree also. On a big monitor screen the image is deformed. All people looks compressed. The original image you can see here; the result in the website you can see here. I tried this with an image with 1920 and 820 pixels and an image with 1920 and 1080 px. Wordpress minimizes the pixels of the 1920 x 1080 pixels till 1920 x 820 px. How can I fix this? Kind regards Hans Hagmeijer