If I go to app -> src -> main -> res and right click on it, there should be "Image Assets" to choose so I can customize my app icon. But as you can see there isn't.

Can someone tell me the reason why?

Android studio Project**

Solution 1: Nayan Sarder

Open android studio File->Open folder->navigate to your flutter project and select the Android folder. Wait for it to sync, then navigate inside the Android studio folder and find the res folder right click on it and choose New you will see the Image asset studio. Maybe this can help you.

Solution 2: aminrd

In order to change your app icon, I highly suggest to choose flutterlauncher_icon package. It is pretty simple and straight forward to change your applications's icon for Android and iOS simultaneously. Just add it to your dependencies and put your Icon.png file in assets/icon/icon.png :

  flutter_launcher_icons: "^0.7.3"

  android: "launcher_icon" 
  ios: true
  image_path: "assets/icon/icon.png"

Anyhow, you should be able to find the current app icons in ./android/app/src/main/res/mipmap-***/ folders.

Solution 3: Suryakant Bharti

That option will appear when you right click the "res" folder.

So, find "res" folder first.