gistfile1.php

<?php
/**
 * Add specific thumbnail size for posts and pages.
 */
function wysija_set_thumbnail_size() {
  $model_config = WYSIJA::get( 'config', 'model' );
  
  // Holds arry of thumbnail sizes
  $sizes = $model_config->getValue('thumb_size');
  
  add_image_size( 'single-post-thumbnail', $sizes['width'], $sizes['height'], true );
}
add_action( 'after_setup_theme', 'wysija_set_thumbnail_size' );