Есть много способов достичь этого, один из них такой:
new Center(
child: new Container(
height: 300.0,
width: 300.0,
decoration: BoxDecoration(
shape: BoxShape.circle,
border: new Border.all(color: Colors.blue, width: 4.0),
),
child: new Stack(
children: <Widget>[
Container(
decoration: BoxDecoration(
shape: BoxShape.circle,
image: DecorationImage(
fit: BoxFit.cover,
image: NetworkImage(
'https://images.unsplash.com/photo-1535593063927-5c40dee9cb83?ixlib=rb-0.3.5&ixid=eyJhcHBfaWQiOjEyMDd9&s=3bfc6e7c6043924de9c4746bef6dc969&auto=format&fit=crop&w=500&q=60',
))),
),
ClipRect(
child: Align(
alignment: Alignment.centerLeft,
widthFactor: 0.5,
child: Container(
decoration: BoxDecoration(
shape: BoxShape.circle,
image: DecorationImage(
fit: BoxFit.cover,
image: NetworkImage(
'https://images.unsplash.com/photo-1535603863228-ded98173a95d?ixlib=rb-0.3.5&ixid=eyJhcHBfaWQiOjEyMDd9&s=78dee486ac6c9bffda623b83a36ecb1f&auto=format&fit=crop&w=500&q=60'),
)),
),
),
),
],
),
),
)
Результат