Images pixelated

Hi,

If you follow the link to our frontpage you can see the few articles at the top have perfectly clear photos, however scrolling down you find most of the images are pixelated. I have no idea what causes this. One of the articles is the same as the featured article, and one image is clear and the other is pixelated. Removing the image and adding it again as a “featured image” does not work. How can I upload the images so they are not pixelated? They are not small photos: 16.6MB

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