To use a background picture do this:

First you want to disable your ground plane.

Go to Display > Guides > and Uncheck Ground Plane.

Then you want to import the background picture you want to use.

Go to File > Import > Background Picture.

Navigate to the folder that has the background picture you wish to use and select your picture.

When the prompt appears to set the background ratio select yes or no according to your desires.

Your background picture should now appear.

You can set up your scene now, but before you render anything be sure your render setting are set to use your background picture.

Do this:

Go to Render > Render Settings

On the Firefly tab under Options Render Over: Click on that down pointing arrow and choose from the choices choose "background picture". After that at the bottom choose "Save Settings"

That is it!

I hope this helps!!

The Artistic Cat