Programmatically Render Image Style for Image Field with Drupal

Drupal 6

Here is a function that you can pass an imagecache preset name and a drupal image file path to and it will try to build a derivative image from the preset name and return to you the path to the derived image:

function tf_imagecache_image_path ($presetname, $file_system_image_path) {
	
  if (!$preset = imagecache_preset_by_name($presetname)) { return; }
  
  $src = imagecache_create_path($presetname, $file_system_image_path);
  
  if (!file_exists($src)) {
    if (!imagecache_build_derivative($preset['actions'], $file_system_image_path, $src)) {
      watchdog("tf","tf_imagecache_image_path - imagecache_build_derivative failed - (" . $preset['actions'] . ") ($file_system_image_path) ($src)",WATCHDOG_WARNING);
    }
  }
  
return $src;
}

Here is an example usage using a CCK Filefield Image field I made called 'field_event_main_image':

$filepath = $node->field_event_main_image[0]['filepath'];
$event_img_src = tf_imagecache_image_path("my_imagecache_preset_name",$filepath);
$event_img_alt = $node->field_event_main_image[0]['data']['alt'];
$event_img_title = $node->field_event_main_image[0]['data']['title'];
$event_img = theme_image($event_img_src,$event_img_alt,$event_img_title);
print $event_img;

Drupal 7

In D7 it is easier to accomplish this. In the following example we'll take an image located at sites/default/files/clothing/hat.JPG and print out the 'thumbnail' for it.

$image = array(
  'style_name' => 'thumbnail',
  'path' => 'public://clothing/hat.JPG',
  'alt' => 'A Picture of a Hat',
  'title' => 'A Cool Hat'
);
print theme('image_style', $image);

The 'thumbnail' image style is a default image style that comes with D7, replace it with your custom image style's machine name to use custom image styles (a.k.a. image cache presets in D6).