title: WordPress grayscale specific image size
body: <p><strong>What to do:</strong> add a new image size and grayscale this specific size.</p>
  
  <p><strong>What I've done so far:</strong>
  I've been searching around and found this helpfull URLs:
  <a href="http://www.wpbeginner.com/wp-tutorials/how-to-grayscale-images-in-wordpress/" rel="nofollow">wpbeginner</a>, <a href="https://coderwall.com/p/z6l8ba" rel="nofollow">Ryan Berry</a>. The second solution creates a grayscale image for each size on every new image and is nearly what I'm searching for.</p>
  
  <p><strong>The Problem:</strong> I know how to add a new image size and the above functions creating grayscale images. What I don't know is how to add this filter <em>just</em> to a specific size.</p>