Hi, as you can see in the URL provided, the large image at the top is of bad quality, while the other images in this blog post seem to look good after resizing. How can I improve this?

Or maybe is it possible to show the featured image full width (as full as the green newsletter bar at the top)?

If there is no solution, where can I change the look of the single post in order for the featured image to show next to the right sidebar?

Thanks for helping! 

Ellen