You can set the background image to an activity by setting android:background xml attributes as followings: (Here, for example, Take a LinearLayout for an activity and setting a background image for the layout(i.e. indirectly to an activity)).
Mobdro is 100% legal in USA. You donât need to worry about it. Up to now, there is no legal issues regarding Mobdro application. Mobdro streaming application is quite familiar to UK and US people.
Respect their terms and conditions and use the Mobdro software. And there is a minor possibility of live streams from the pirated sources where you may end up in legal troubles. To avoid getting caught when streaming pirated content online, you need to get connect to a VPN server where you can feel safe and protected.