A GUI is in the works for changing settings from within VS, but currently it requires manually updating the config.dat file located in the installation directory.
Revisions:
10/19/2012: v1.2 - Fixed a bug where some text/line highlighting was being obscured or changing color. The image is now being drawn in a lower "layer" to keep these conflicts from happening.
Description
    ====================================================================
    The Visual Studio 2012 Text Editor backround changer extension allows
    you to customize the background of the normally boring text editor. 
    By default, when the extension is installed, a predefined image directory
    and default image has been setup. These can be easily changed by modifying
    the config.dat file within the extension's installation directory. The most common
    installation location is:
    C:\Users\<User Name>\AppData\Local\Microsoft\VisualStudio\11.0\Extensions\
    
    Config.dat
    =====================================================================
    This is the primary method for manually adding/changing the background 
    image. By changing the contained parameters, the associated changes
    will appear immediately the next time a new text editor window is opened. 
    There's no need to restart Visual Studio. 
    Config.dat Parameters
    =====================================================================
    img_directory:  This is the location that the extension will seearch 
                    when looking for the image defined in the img_name 
                    property.
                    Default: The default location is [VSIXInstallDirectory]\Images 
                    where [VSIXInstallDirectory] is interpreted by the extension to be 
                    whatever folder the extension was installed into. 
                    Any folder can be listed as long as the proper read/write 
                    permissions are available on the folder.
    img_name:       The specific image name. Only .jpg and .png formats have been 
                    tested, so your mileage may vary.
    opacity:        A 0.0 - 1.0 value which sets the images opacity. An opacity of 1.0
                    will have no opacity. 
                    Default: The default opactiy is set at 0.35. This seems to be a
                    good blend to allow text to flow over the image without much 
                    interference
    location:       There are 5 discrete position the image can be put. They are, 
                    TopLeft, TopRight, BottomLeft, BottomRight and Center. 
                    Default: The default watermark location is BottomRight.
    fill:           NOTE: This setting is still a little iffy. As of this version, 
                    there are 4 discrete values that can be set, all corresponding 
                    to the System.Windows.FrameworkElement's Stretch property. 
                    None, Uniform, Fill, UniformToFill. 
                    Default: The default value is None.