D7: Using the Picture module in code

your_module.module

<?php
/**
 * Helper function.
 *
 * Prepares the necessary array for use with theme_picture.
 *
 * @param array $variables
 *   The image array loaded from field_get_items.
 *
 * @param string $group
 *   The breakpoint group to use for breakpoint mappings.
 *
 * @return array
 *   A fully prepared picture array.
 */
function your_module_prepare_picture($variables, $group) {
  $mappings = picture_mapping_load($group);

  $picture_arr = array(
    'item' => $variables[0],
    'uri' => $variables[0]['uri'],
    'height' => $variables[0]['metadata']['height'],
    'width' => $variables[0]['metadata']['width'],
    'breakpoints' => $mappings->mapping,
    'style_name' => 'original',
  );

  return $picture_arr;
}


// Usage in say... hook_preprocess_node().
function your_module_preprocess_node(&vars) {
  $some_img_field = field_get_items('node', $node_obj, 'field_some_field_name');
  $some_img_array = your_module_prepare_picture($some_img_field, 'breakpointgroup');
  if (!empty($some_img_array)) {
    $variables['some_image'] = theme('picture', $some_img_array);
  }
}

// Now in your template:
<?php print render($some_image); ?>
?>