how to use image styling on a image url field
Try using the Imagecache External drupal module: https://drupal.org/project/imagecache_external

In your template file you could then call (from the drupal project example) and replace 'path' with your path from your url field:

&lt;?php 
  print theme('imagecache_external', array(
    'path' =&gt; 'http://larsjung.de/h5ai/sample/screenshots/h5ai-v0.7-icons.png', 
    'style_name'=&gt; 'thumbnail'),
  );
?&gt;