When you submit the form, the value is presented with debug_dump(), so strings are quoted and array values listed.

:
: - Stop the submitted form from bring processed
: - Don't process the form, redirect to another page (e.g. login after session timeout), and remember the values for when they return.

or back to examples 

<?php
	$field = new form_field_image($form, 'Image');
	$field->max_size_set('The image file cannot be bigger than XXX.', 1024*1024*1);
	$field->min_width_set('The image must be more than XXX wide.', 300);
	$field->max_width_set('The image must not be more than XXX wide.', 500);
	$field->min_height_set('The image must be more than XXX high.', 300);
	$field->max_height_set('The image must not be more than XXX high.', 500);
	$field->required_error_set('The image is required.');
	$field->file_type_error_set('The image file has an unrecognised file type (XXX).');
	// $field->multiple_set(true);
?>