jamiemagique

// Get image settings array from a field on the node
$image = field_get_items('node', $node, 'INSERT_IMAGE_FIELD_MACHINE_NAME');

// Picture settings
$picture_mapping = picture_mapping_load('INSERT_PICTURE_MAPPING_MACHINE_NAME');
$fallback = 'INSERT_IMAGE_STYLE_MACHINE_NAME';
$breakpoints = picture_get_mapping_breakpoints($picture_mapping, $fallback);

// Create a new variable to pass through to the node template
$variables['INSERT_NEW_VARIABLE_NAME_TO_PASS_TO_NODE_TEMPLATE'] = theme('picture', array('uri' => $image[0]['uri'], 'style_name' => $fallback, 'breakpoints' => $breakpoints));