There is no "default background" of an Android app. It all depends on the theme of the device.
You have 2 options:
1) Override the background of your app's activity to your own color or drawable. You can do this using the android:windowBackground attribute of your theme. See the docs. Example:
<style name="MyTheme" parent="android:Theme.Holo.Light.DarkActionBar">
<item name="android:windowBackground">@drawable/grey_background_pattern</item>
</style>
2) You should be using transparent pixels in your image instead of white. This way your image will better fit with the other colors/backgrounds of your app.