You could create a custom renderer for images and use there for example a CachedNetworkImage.

'img': (renderContext, child, attributes, _) => CachedNetworkImage(
  width: attributes['width'],
  placeholder: (context, url) => Container(),
  imageUrl: attributes['src'],
  errorWidget: (context, url, error) => Container(),
);

You could create a custom renderer for images and use there for example a CachedNetworkImage.

'img': (renderContext, child, attributes, _) => CachedNetworkImage(
  width: attributes['width'],
  placeholder: (context, url) => Container(),
  imageUrl: attributes['src'],
  errorWidget: (context, url, error) => Container(),
);

You forgot to parse to double width.

With the new image loading API (see README) you can do this:

networkSourceMatcher(): networkImageRender(altWidget: (_) => FlutterLogo()),

This will render the Flutter logo as a placeholder.