Full screen background behind ProgressBar & maintain aspect ratio

[mcfc4heatons]

Newbie question, I'm trying to work out how to have a full screen background behind ProgressBar, and need background image to keep its aspect ratio. As I understand it it might not be possible to maintain aspect ratio with background image?

in activity_main.xml I have

        <ProgressBar
            android:id="@+id/progressBar1"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_gravity="center"
            android:background="@mipmap/splash"
            android:indeterminate="false"
            android:padding="120dp"
            android:textAlignment="center" />

in java/MainActivity show progressbar:

if (isNetworkStatusAvialable(getApplicationContext())) {
    mWebView.setWebViewClient(new com.myapp.webview.MyAppWebViewClient() {

        @Override
        public void onPageStarted(WebView view, String url, Bitmap favicon) {
            findViewById(R.id.progressBar1).setVisibility(View.VISIBLE);
        }

then hide when ready:

                @Override
                public void onPageFinished(WebView view, String url) {
                    //hide loading image
                    findViewById(R.id.progressBar1).setVisibility(View.GONE);
                    //show url in webview
                    findViewById(R.id.web_view).setVisibility(View.VISIBLE);
//Close loading
                    mySwipeRefreshLayout.setRefreshing(false);
                }
            });

It does the job except background image is just stretched to fit and doesn't always look great as it loses its aspect ratio.