We all know about Microsoft's latest Office suite "Office 2013" (codename Office 15) which is currently under development and a free Customer Preview version of Office 2013 can be downloaded using following link:
Download Free Customer Preview Version of Microsoft Office 2013
Microsoft has made many changes in this new version. The most noticeable change in Office 2013 is its UI. Microsoft has removed Aero glass from Office 2013 to match its interface with Windows 8. Microsoft has also removed colored themes from Office 2013 and has introduced new Metro style background images.
There are currently 6 background images available in Office 2013 as shown in following image:

By default no background image is set in Office 2013 programs but you can set a background image using Office Options. Open any Office program, click on File menu and then click on Options present in left sidebar.
Once you open Options window, you can change the background image using the drop-down box given in Office Background section as shown in following image:

Unfortunately, this method is not working for many people. Every time they try to change the background image and click on OK button, Office automatically resets the background image to None. There is no error message or instruction given to fix this issue.

In fact I also faced this problem in Office 2013 and after a little troubleshooting I found the solution.
The solution is sign in using a Windows Live ID and you'll be able to change or set background image in Office 2013 programs.
To sign in, click on "Sign in" link given below the titlebar buttons or click on File menu and then click on Account option present in left sidebar. Enter your Windows Live credentials and once you complete the sign in process, you'll be able to change background image.
Now you can change the background image using Options window or just click on your username or avatar given below titlebar buttons and select "Account settings" option. The same can also be accessed by clicking on File menu and then Account option in left sidebar.

As you can see in the above screenshot, you can quickly change the background image. It also shows live preview as soon as you hove mouse cursor over the given background images.
So the bottom line is that the background images can only be set or changed if and only if you are signed in using a Windows Live ID otherwise you'll not be able to use them.
In future if you want to sign out from Office 2013, open Account settings and click on "Remove" link given in user information section.
Keep in mind that signing out from Office 2013 will again reset background image to none. Its difficult to understand why a simple option to change background image is available to signed in users only? And if Microsoft really wants Office users to sign in to use this feature, they should put some kind of information or messages so that users can know how to use it. If a user is not signed in, the option to change background image should be greyed out.
Posted by: Vishal Gupta | Categories: Microsoft Office, Troubleshooting
Punnet
Thank u so much vg. I was having the same problem. Your solution worked like a charm. :D
Vic
You are genius Vishal. :)
Samantha
cool. I did not know about these backgrounds. thnx for posting it.
Whiston
I was already signed in so never noticed it.
Mayank
But I am not at all happy with 2013 series.
Few days back I was working in Word.
I had to insert multiple images into word and I was annoyed at the way new Word 2013 handles it. When I insert multiple images, instead of placing them side by side and moving down if page was small, Word 2013 does it very differently.
Theyhave surely added many new functionalities in Word 2013 but it is of no use if a user gets annoyed with the basic things.
They have really messed up the basic functions of Word.
Kyle
@To all who didn't know about this,
you would all be surprised that I've already known about this ever since I downloaded Office 2013 [beta] on the first day it came out. I've signed in with my Windows Live ID when the [Getting Started] box appeared. That time, as what VG stated, the Office 2013 background was blank. By the time I sign in, the GS box automatically asked me "what design would you like for your [name of Office product]?"
@Mayank, I'm not sure what you mean but when I inserted and edited my photos, I've encountered no problems nor anything annoying.
imo, The fact that one has to use a Windows Live ID to customize a basic thing has pretty much turned Office 2013 into something similar to back in Windows 8 testing build days.
terabytex4
and the offline user or computer without internet connection just got blank background image??
Thats an annoying thing!!
W. Michael Biklen
I would respectfully request that Microsoft restore the ability to create color schemes for Office products just as was the case in Office 2010 and earlier. Likewise, I would respectfully request that Office products restore the ability to insert images directly from a scanner as was the case of Office 2007 and earlier.
Kim
Worked like a charm....thank you.