title: Add background image to a custom view
body: <p>I'm developing a small android project of a 'virtual blackboard'. I'm able to draw on my mobile/tablet and the lines are stored in a server to be reproduced in future (lines are sent through socket)</p>
  
  <p>All this stuff is done by using a custom View. Now, I need to add the option of changing the view's background dynamically. By now it does not matter how I'll get this image (from gallery, sent by another app, etc). It's enough for the moment to have something (a file, an inputstream, a Bitmap) and set it as my view's background and then be able to draw on it (I'm still not worrying about sizing, scaling, etc)</p>
  
  <p>I've read some stuff about loading images from resources and assets but none of them apply to my case (IMO) since images are not part of my app.</p>
  
  <p>Any suggestion on how I should proceed?</p>