Just create a background.png file you want to use and copy it to
/BeatX/background/background.png folder (BeatX folder is created on your external storage on first game start)
If you use landscape mode create backgroundRot.png file and copy it to /BeatX/background/backgroundRot.png folder.
I recommend to resize the image to exactly the same size as your display resolution - this will limit the the load times to minimum and will give you the best image quality possible.
If the image is not native resolution of the device it will be stretched/resized on run time.
You can resize your image using free tools:
http://www.irfanview.com/ (Win compatible)