There is actually an issue filed in GitHub related to this.
In your case, it seems that you treat a Flutter project as a regular Android project. AFAIK, Flutter projects are not Android projects.
So one of the workaround is also available:
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.
Also take note of this:
don't open the whole project of flutter using Android studio just open
the android folder found inside the flutter project with Android
studio, so the Android studio can treat it as a Android project rather
than flutter project, which will give you all Android features like
the one ur looking for #image asset studio
And you can also check this blog post about changing your Flutter app icons. And this YouTube video. You will only need your desired image for the icon and App Icon Generator.
To summarize what was written in the blog and discussed in the YouTube post, here are the simple steps you need to follow:
Step 1: Visit appicon.co and upload your intended image to convert to icons, select the types you would like to be generated and click “generate”. This will download a zip folder to your system. (You may skip this step if you have all resolutions in place with you.)

Step 2: Unzip the file. You’ll have two folders by the name of “android” and “Assets.xcassets”.

Step 3: Changing icons for Android App.
In your Flutter project, move to android/app/src/main/res which has a lot of folders starting with “mipmap-”. Delete all of them. And replace from your downloaded and unzipped AppIcons folder, everything that’s under android.
