You can provide users with a choice of background images for the LCD screen on their phones. Users can select a background image by choosing Settings > User Preferences > Background Images on the phone.
The image choices that users see come from PNG images and an XML file (called List.xml) that are stored on the TFTP server used by the phone. By storing your own PNG files and editing the XML file on the TFTP server, you can designate the background images from which users can choose. In this way, you can provide custom images, such as your company logo.
http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/7906g_7911g/5_1/sip/english/administration/guide/1106cst.html