Generating thumbnails on the fly with WordPress

One advantage of Drupal’s resizing images over WordPress – WordPress’s resized images are generated at upload time only, whereas Drupal’s will be generated automatically on page load if the thumbnail doesn’t already exist.

You could put this function into your WP template – given an image url $image_url, it checks for the address of its thumbnail $thumb_url. If it doesnt find it, then it generates a thumbnail of size $xdim x $ydim

function maybe_generate_userarticlethumb($thumb_url, $image_url, $xdim, $ydim) {  
  
  /* There might be quicker ways to check if url exists than file_get_contents */
  if(!@file_get_contents($thumb_url)) {   
    $imagepath = str_replace(array($_SERVER["HTTP_HOST"], 'http://', 'https://'), array($_SERVER["DOCUMENT_ROOT"], '', ''), $image_url);
    $thumbpath = str_replace(array($_SERVER["HTTP_HOST"], 'http://', 'https://'), array($_SERVER["DOCUMENT_ROOT"], '', ''), $thumb_url);
    $image = wp_get_image_editor($imagepath);
    if ( ! is_wp_error( $image ) ) {
       /* Use any of WP's image manipulation functions here */
       $image->resize( $xdim, $ydim, true );
       $image->save($thumbpath);
    }
  }
}  

Then just call maybe_generate_userarticlethumb($thumb_url, $image_url, $xdim, $ydim) in your template, and print $thumb_url where you need to.