Managing activity background image
<p>I want to set a 320x240 image as background of the activity in two ways.</p>

<p>First one is to set this image in the center on the activity, not enlarging it - keeping the original size of the image.
Second one, also in the center, but enlarning to 500x400.</p>

<p>Using simply <code>android:background="@drawable/background</code> enlarges the image to the activity size.
What do I have to do to achieve these effects?</p>
