Restrict WordPress Media image upload dimensions

restrict_wp_image_upload_dimensions.php

<?php

function restrict_image_upload_size($file) {
  if(!stristr($file['type'], 'image')) { // only run on images
    return $file;
  }

  $img     = getimagesize($file['tmp_name']);
  $minimum = array('width' => '1024', 'height' => '768');
  $width   = $img[0];
  $height  = $img[1];

  if($file['error'] != '0') { // there's already an error
    return $file;
  }

  if(defined('WP_IMPORTING')) {
    return $file;
  }

  if($width < $minimum['width'] || $height < $minimum['height']) {
    $file['error'] = "This image must be a minimum of {$minimum['width']}px x {$minimum['height']}px; the uploaded image is only {$width}px x {$height}px";
  }

  return $file;
}
add_filter('wp_handle_upload_prefilter', 'restrict_image_upload_size');