title: update and regenerate thumbnails for wordpress multisite
body: <p>For my multisite installations (with over 1500 sites on one of them) I am looking for the most efficient way to change the thumbnailsite to 300x300. On a single site you can easily do this by changing the mediasettings and then running the regenerate thumbnails plugin. I don't want to do this manually for all the sites so I'm thinking about how to do this by code.</p>
  
  <p>To set the thumbnailsize is a part which I can figure out. I'll just make a loop that loops through all the sites in the network and use the code to update the default thumbnailsettings:</p>
  
  <pre><code>if (function_exists('add_theme_support')) { 
      add_theme_support( 'post-thumbnails' ); set_post_thumbnail_size( 300, 300, true ); // default thumbnail size 
  }
  </code></pre>
  
  <p>But I can't figure out how to do the regenerating since I can't really find advice on how to code this. When I search for this I just find developers recommending the regenerate thumbnails plugin. Also with the code I'm a little bit skeptical it'll become too heavy for my wpmu.</p>
  
  <p>Is there something code/plugin that can do what I want ? (regenerate thumbnails for the whole network ?)</p>