gistfile1.php

<?php
	private static $image_alignment_options = null;

	//....
	new TextField('CaptionText', _t('HtmlEditorField.CAPTIONTEXT', 'Caption text')),
	DropdownField::create(
		'CSSClass',
		_t('HtmlEditorField.CSSCLASS', 'Alignment / style'),
		$this->getImageAlignmentOptions()
	)->addExtraClass('last')
	//....
	
	public function getImageAlignmentOptions() {
		$options = Config::inst()->get('HtmlEditorField', 'image_alignment_options');
		if ($options) {
			return $options;
		} else {
			return array(
				'leftAlone' => _t('HtmlEditorField.CSSCLASSLEFTALONE', 'On the left, on its own.'),
				'center' => _t('HtmlEditorField.CSSCLASSCENTER', 'Centered, on its own.'),
				'left' => _t('HtmlEditorField.CSSCLASSLEFT', 'On the left, with text wrapping around.'),
				'right' => _t('HtmlEditorField.CSSCLASSRIGHT', 'On the right, with text wrapping around.')
			);
		}
	}