When you upload a large image and then insert it into a post, if you insert it as a thumbnail or a medium, wordpress resizes and resamples the image. If you insert it as “full-sized” wordpress inserts it into the image and simply passes the size to be displayed on to the browser and the browser then renders it at the size needed.
To get high quality images in the post, resize and resample the images to the exact width needed by your particular theme in your image editing program and then upload that image. That way you aren’t relying on wordpress or the browser to size the image. In the case of the theme you are using that is 509px in width.