try this:
appBar: AppBar(
title: Row(
mainAxisAlignment:MainAxisAlignment.center,
children:[
Text(
'my app name',
style: TextStyle(
fontFamily: 'OpenSansBold',
fontSize: 26.0,
),
),
Icon(
Icons.audiotrack,
color: Colors.green,
size: 30.0,
),
]
)
),
also, you can change mainAxisAlignment for aligning these items or change their order to show the icon at the start or the end of the text. The result is as follow:

Update
if you want to add an image instead of an icon, try this:
appBar: AppBar(
title: Row(
mainAxisAlignment:MainAxisAlignment.center,
children:[
Text(
'my app name',
style: TextStyle(
fontFamily: 'OpenSansBold',
fontSize: 26.0,
),
),
Image(image: AssetImage('your directory'));
]
)
),
note that first, you should add your image to the asset section of pubspec.yaml file as follow:
flutter:
assets:
- Your directory
for more info see this.