Upload image android to webservice asp.net

Good morning, I'm trying to upload an image of Android using an ASP MVC controller method but without success, could you give me some guidance?

Here I declare the Retrofit method

@POST("prueba")
Call<JsonElement> prueba(@Query("img") Bitmap img);

Next, the Android method by which I try to upload the previously loaded image into an ImageView

public void prueba() {
    MuniWebServices services = ApiMuniWeb.getApi().create(MuniWebServices.class);
    Bitmap bm = ((BitmapDrawable) img1.getDrawable()).getBitmap();

    Call<JsonElement> call = services.prueba(bm);

    call.enqueue(new Callback<JsonElement>() {
        @Override
        public void onResponse(Call<JsonElement> call, Response<JsonElement> response) {
            Toast.makeText(getContext(), response.message(), Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onFailure(Call<JsonElement> call, Throwable t) {
            Toast.makeText(getContext(), t.getMessage() + " - " + t.toString(), Toast.LENGTH_SHORT).show();
        }
    });
}

and finally this is the method in the ASP.MVC driver

Public Function prueba(img As Bitmap) As Codigo

    Try
        Dim format As Imaging.ImageFormat = Imaging.ImageFormat.Jpeg
        Dim fs As FileStream = New FileStream(System.Web.Hosting.HostingEnvironment.MapPath("~\paginas\img_reclamos") + "/prueba.jpg", FileMode.Create)
        img.Save(fs, format)
        fs.Close()
        fs.Dispose()

        Return obtenerCodeOK()
    Catch ex As Exception
        Return obtenerCodeError(ex.Message.ToString)
    End Try
End Function

The methods are connected correctly and the return of them is correct tmb, the problem is that I create an image in the server folder completely empty as the one shown below