From document Add a splash screen to a .NET MAUI app project,we know that

On Android 12+ (API 31+), the splash screen shows an icon that’s
centred on screen.

On Android 12+ (API 31+),it’s impossible to make image to full screen in splash screen. It is only possible to customize it: icon, window background, exit animation. Please check document: Splash screens on developer.android.com.

However, there is a workaround to make the splash screen to transparent, and then you can create a splashScreen with ContentPage， add images in the ContentPage.

Step 1. Create styles.xml at Platforms/Android/Resources/values folder, and add the following code to customize style. And make sure the build action of styles.xml is AndroidResource.

<?xml version="1.0" encoding="utf-8" ?>
<resources>
<style name="MyTheme.Splash" parent ="MainTheme">
<item name="android:windowIsTranslucent">true</item>                  
<item name="windowNoTitle">true</item>
<item name="windowActionBar">false</item>
<item name="android:windowSplashScreenAnimatedIcon">@android:color/transparent</item>
<item name="android:windowSplashScreenAnimationDuration">0</item>
</style>
</resources>

Step2. Open the MainActivity.cs, change the theme to Theme = "@style/MyTheme.Splash".

[Activity(Theme = "@style/MyTheme.Splash", ...)]
public class MainActivity : MauiAppCompatActivity
{
}

Step3. please create a Contentpage called SplashScreen and add your image to it. And open the Mainpage after several seconds by Task.Delay() and Application.Current.MainPage = new MainPage ();.
Just like the following code.

protected override async void OnAppearing()
{
    base.OnAppearing();
 
   await Task.Delay(1000).ContinueWith(t => {
       MainThread.InvokeOnMainThreadAsync(() => {
           Application.Current.MainPage = new MainPage ();
       });
    }); 
}

Step4. open the App.xaml.cs, change the MainPage to the SplashScreen.

  public partial class App : Application
  {
      public App()
      {
          InitializeComponent();
 
          MainPage = new SplashScreen();
      }
  }