ClipRRect

String _imageUrl = 'https://images.unsplash.com/photo-1565898835704-3d6be4a2c98c?fit=crop&w=200&q=60';

ClipRRect(
  borderRadius: BorderRadius.circular(50),
  child: SizedBox(
    width: 100,
    height: 100,
    child: Image.network(_imageUrl, fit: BoxFit.cover,),
  ),
)