Automatically Set WordPress Thumbnail Sizes When Switching Themes

Right now, one of my main projects at work is a totally new theme for the official news outlet for a major corporation. Along with a million other changes, one thing that needs to happen ASAP after switching to the new theme is that the image thumbnail sizes – controlled through Settings › Media in wp-admin – need to be changed.

Of course, being the lazy, automation-obsessed developer that I am, I wanted to find a way to automatically set these thumbnail sizes the instant we changed themes. Fortunately, WordPress fires the after_switch_theme as soon as the new theme is set; with that action, we can add something like the following to our new theme’s functions.php file:

 * Change the default image sizes within WordPress.
 * In the $image_sizes array, the key corresponds to an image size
 * while the value is either null (no change) or an array
 * containing two values: a width and a height.
function mytheme_set_image_sizes() {
  // Obviously change these to suit your needs.
  $image_sizes = array(
    'thumbnail'    => array( 200, 200 ),
    'medium'       => null,
    'medium_large' => null,
    'large'        => array( 1024, 1024 ),

  foreach ( $image_sizes as $size => $dimensions ) {
    if ( null === $image_sizes || 2 !== count( $dimensions ) ) {

    // Update the width and height based on $dimensions.
    update_option( $size . '_size_w', $dimensions[0] );
    update_option( $size . '_size_h', $dimensions[1] );
add_action( 'after_switch_theme', 'mytheme_set_image_sizes' );

Now, as soon as we switch themes, we’ll automatically change the sizes stored in our wp_options table, based on our $image_sizes array.

With great power…

Of course, just because you can do something doesn’t mean that you should. If you’re building a new, custom theme, automatically setting thumbnail sizes like this might be super useful. If you’re releasing something publicly, a technique like this starts bordering on “eh, maybe you’re overstepping your bounds as a theme developer.” I’m not here to make that call, I just came here to chew gum and build themes (and I’m all outta gum).

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