How To Make Background Image Fill And Fit The Screen

I've started learning about drawing things to the screen with Surface view and canvases, but I've run into a problem. At the moment, all I want to do is draw a background image to the screen, and have it fit the screen, regardless of size or resolution. I've figured out how to draw the image just fine, but it doesn't fit the screen at all.

Here is my situation: I'm developing on an Archos 5 internet tablet. It has a 4.8 inch screen and a resolution of 800x480 - putting it in the "Large" class of screens and "Medium" class of pixel density for Android. My background image is 800x480 - my thinking was that it would be displayed perfectly on my device, and scaled down on smaller screens.

But this isn't happening. When I place my image in the "medium" density drawables folder, the image seems "zoomed in" and I can't see it all on my device. And when I emulate the app with different screen sizes, it's all over the place.

I'm drawing my image in a Panel which extends Surface View. Here is the code:

Code:
package com.plant.drawing;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.view.SurfaceView;
import android.view.SurfaceHolder;
import android.content.Context;

[Code] ....

In my layout, I set this panel's properties of layout height and width to "wrap content." I'm really new to Surface views and the like, so I'm sure I'm simply not doing something - but what can I do to make this image fill the background of any size screen? The image is a .png, by the way.