image method
Implementation
Future<Uint8List?> image(String href) async {
final Uint8List? cached = _images.get(href);
if (cached != null) return cached;
final UEpubBook? book = _book;
if (book == null) return null;
final Uint8List data = await book.resource(href);
if (data.isEmpty) return null;
_images.put(href, data);
return data;
}