return Image.network(
        mediaUrl,
        fit:BoxFit.contain,
        width:MediaQuery.of(context).size.width * 0.9,
        height: MediaQuery.of(context).size.height - 200,
        loadingBuilder:(BuildContext context, Widget child, ImageChunkEvent? loadingProgress) {
             if (loadingProgress == null){
               return child;
             }
             return Center(
               child:Padding(
                 padding: EdgeInsets.all(20),
                 child: CircularProgressIndicator(
                   value: loadingProgress.expectedTotalBytes != null
                       ? loadingProgress.cumulativeBytesLoaded / loadingProgress.expectedTotalBytes!
                       : null,
                 ),
               ),
             );
        },
);

通过loadingBuilder实现图片加载时的进度显示。