if (Environment.getExternalStorageState().equals(
                Environment.MEDIA_MOUNTED)) {

<receiver
    android:enabled="true"
    android:exported="false"
    android:name=".ExternalStorageInfoReceiver">
    <intent-filter>
            <action android:name="android.intent.action.MEDIA_MOUNTED"/>
    </intent-filter>
  </receiver>

public class ExternalStorageInfoReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {
       //change wallpaper here....
    }
}

<receiver android:name=".YourBroadcastReceiver">
      <intent-filter>
           <action android:name="android.intent.action.ACTION_EXTERNAL_APPLICATIONS_AVAILABLE" />
           <category android:name="android.intent.category.DEFAULT" />
      </intent-filter>
</receiver>