How to add your own Sitecore Desktop Background Image

You can change your Sitecore desktop background through this option in the control panel, 

Now, when you click the option, you get a modal window and there is a list of backgrounds you can select from -

If you want to add your own background image, there is no option to add one here. 

I set about searching the core database without much luck. But, if you inspect the code, you should be able to find that the images are picked from the local file system -

Here is the concerned folder in the web root -

Now, if you want to add your custom image, just copy over a file with extension .jpg over to the C:\inetpub\wwwroot\sc1011sc.dev.local\sitecore\shell\Themes\Backgrounds location.

Now, reopen the change desktop background window and you must be able to see the title of the jpg file name -

Select the one you want to setup as background image. Apply the changes and check the desktop to see the change take effect -