how to start fresh and add custom backgrounds
Create a folder called "myTheme" in sites/all/themes
Create a file in that new folder called "myTheme.info" 
Copy the sample .info from below into the info file
Now create a folder in your theme called "css"
Now copy style.css from Bartik's css folder into your css folder
Now change the background using CSS
Any other changes you want to make? Copy from bartik into your theme folder, make the change


Sample info file:

name = My Theme
description = My theme is amazing
core = 7.x
engine = phptemplate
base theme = bartik