This plugin includes a "Background image" field (with a button to launch the server browser) in the Table and Cell properties dialogs.
Further explanations at the post in my blog.
Plugin sponsored by Igenti Inc.
Licensed under the terms of any of the following licenses at your choice: GPL, LGPL and MPL.
Backgrounds version 1.3.1:
1. Extract the contents of the file into the "plugins" folder of CKEditor.
2. In the CKEditor configuration file (config.js) add the following code:
config.extraPlugins = 'backgrounds';
All the details and options are explained in the included install doc.