Maps API: PNG for GroundOverlay

Question

I'm creating an app for Android which uses Maps API and I want to use GroundOverlay.

Here is my actual onMapReady:

@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;

    //defining coordinates of image bounds
    LatLngBounds imageMapBounds = new LatLngBounds(
        new LatLng(46.709064, 2.888008),
        new LatLng(46.729014, 2.918522)
    );

    //adding image and coordinates to the GroundOverlay
    GroundOverlayOptions imageMapOptions = new GroundOverlayOptions()
            .image(BitmapDescriptorFactory.fromAsset("map.png"))
            .positionFromBounds(imageMapBounds);

    //adding GroundOverlay to the map
    GroundOverlay map = mMap.addGroundOverlay(imageMapOptions);
}

I've tried other methods found on google but in all cases, this error happens :

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: fr.rm_jln.parilly, PID: 29898
                  com.google.maps.api.android.lib6.common.apiexception.b: Failed to decode image. The provided image must be a Bitmap.
                      at com.google.maps.api.android.lib6.impl.h.a(:com.google.android.gms.dynamite_dynamitemodulesb@[email protected] (040408-194189626):9)
                      at com.google.maps.api.android.lib6.impl.o.a(:com.google.android.gms.dynamite_dynamitemodulesb@[email protected] (040408-194189626):7)
                      at com.google.maps.api.android.lib6.impl.bj.<init>(:com.google.android.gms.dynamite_dynamitemodulesb@[email protected] (040408-194189626):37)
                      at com.google.maps.api.android.lib6.impl.bc.a(:com.google.android.gms.dynamite_dynamitemodulesb@[email protected] (040408-194189626):539)
                      at com.google.android.gms.maps.internal.l.onTransact(:com.google.android.gms.dynamite_dynamitemodulesb@[email protected] (040408-194189626):98)
                      at android.os.Binder.transact(Binder.java:499)
                      at com.google.android.gms.internal.maps.zza.transactAndReadException(Unknown Source)
                      at com.google.android.gms.maps.internal.zzg.addGroundOverlay(Unknown Source)
                      at com.google.android.gms.maps.GoogleMap.addGroundOverlay(Unknown Source)
                      at fr.rm_jln.parilly.DisplayPathActivity.onMapReady(DisplayPathActivity.java:100)
                      at com.google.android.gms.maps.zzak.zza(Unknown Source)
                      at com.google.android.gms.maps.internal.zzaq.dispatchTransaction(Unknown Source)
                      at com.google.android.gms.internal.maps.zzb.onTransact(Unknown Source)
                      at android.os.Binder.transact(Binder.java:499)
                      at fr.b(:com.google.android.gms.dynamite_dynamitemodulesb@[email protected] (040408-194189626):20)
                      at com.google.android.gms.maps.internal.bg.a(:com.google.android.gms.dynamite_dynamitemodulesb@[email protected] (040408-194189626):5)
                      at com.google.maps.api.android.lib6.impl.be.run(:com.google.android.gms.dynamite_dynamitemodulesb@[email protected] (040408-194189626):5)
                      at android.os.Handler.handleCallback(Handler.java:761)
                      at android.os.Handler.dispatchMessage(Handler.java:98)
                      at android.os.Looper.loop(Looper.java:156)
                      at android.app.ActivityThread.main(ActivityThread.java:6523)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:942)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:832)

As written in the third line, GroundOverlay only accept .bmp images. The problem is that my image weighs ~15Mo when in .png and over 500Mo when in .bmp. Images in .bmp can't be compressed and I can't resize it smaller if I want a minimum of quality.

So my question is : how can I use GroundOverlay without having a file that is much too heavy ?