How to use .png image as an icon in Flutter
Instead of the icon, you can use a transparent image .png.
In this example, I used the image in the bottom navigation bar and the floating action button.
bottomNavigationBar: BottomNavigationBar(items: [
BottomNavigationBarItem(
icon: Image.asset(
"assets/FlutterAssets_logo.png",
width: 40,
height: 40,
),
label: 'PNG icon 1'
),
BottomNavigationBarItem(
icon: Image.asset(
"assets/FlutterAssets_logo.png",
width: 40,
height: 40,
),
label: 'PNG icon 2'
),
BottomNavigationBarItem(
icon: Image.asset(
"assets/FlutterAssets_logo.png",
width: 40,
height: 40,
),
label: 'PNG icon 3'
),
],
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Image.asset(
"assets/FlutterAssets_logo.png",
width: 40,
height: 40,
),
),

You can read more about Icons in Fluter in this post:
Flutter Basics – How to use Icons in Flutter
Also, you can read more about images here: