DecoratedBox(
  decoration: BoxDecoration(
    image: DecorationImage(
      fit: BoxFit.cover,
      image: AssetImage(img),///例えばここをCachedNetworkImageにしたくてもできない
    ),
    shape: BoxShape.rectangle,
  ),
  child: Container(
    margin: const EdgeInsets.only(top: 30.0),
    decoration: const BoxDecoration(
    ),
  ),
)

CachedNetworkImage(
  imageUrl: img,
  fit: BoxFit.cover,
  ///これが使える
  imageBuilder: (context, imageProvider) => Container(
    decoration: BoxDecoration(
      shape: BoxShape.rectangle,
      image: DecorationImage(image: imageProvider, fit: BoxFit.cover),
    ),
    child: Container(
      decoration: const BoxDecoration(
      ///略
      ),
    ),
  ),
)