All 3?: Responsive resizing of images + image width of actual image for small images + max-width for all larger images

The CSS

Add the following code to your CSS file. That will make the images scalable according to screen size.

Obviously change the class tag if you want to set up just for a specific image class

img{ max-width: 100%; }

img{ -ms-interpolation-mode: bicubic; }

Then you need to run a filter to stop wordpress automatically adding image dimensions to the images you want responsive.

  function remove_wp_width_height($string){
  return preg_replace('/\/i', '',$string);
  }

Then when you need to call the thumbnail so in your template instead of using the_post_thumbnail(); use your new function like this

echo remove_wp_width_height(get_the_post_thumbnail(get_the_ID(),'large'));