Hi Ammar,
Here is a clip of the document linked below that lists all the steps required;
Configuring a Custom Background Image
To create custom background images for the Cisco Unified IP Phone, follow these steps:
Procedure
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Step 1 Create two PNG files for each image (a full size version and a thumbnail version). Ensure the PNG files comply with the format guidelines that are listed in the "PNG File Requirements for Custom Background Images" section.
Step 2 Place the new PNG files that you created in the following folder on the TFTP server for each Cisco Unified CallManager in the cluster:
C:\Program Files\Cisco\TFTPPath\Desktops\320x212x12
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Note Cisco recommends that you also store backup copies of custom image files in another location. You can use these backup copies if the customized files are overwritten when you upgrade Cisco Unified CallManager.
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Step 3 Use a text editor to edit the List.xml file. See the "List.xml File Format Requirements" section for the location of this file, formatting requirements, and a sample file.
Step 4 Save your modifications and close the List.xml file.
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Note When you upgrade Cisco Unified CallManager, a default List.xml file will replace your customized List.xml file. After your customize the List.xml file, make a copy of the file and store it in another location. After upgrading Cisco Unified CallManager, replace the default List.xml file with your stored copy.
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Step 5 To cache the new List.xml file, stop and start the TFTP service by using Cisco Unified CallManager Serviceability or disable and re-enable the Enable Caching of Constant and Bin Files at Startup TFTP service parameter (located in the Advanced Service Parameters).
Creating Custom Background Images
From this doc;
Cisco Unified IP Phone 7970 Series Administration Guide for Cisco Unified CallManager, Release 4.2 (for models 7970G and 7971G-GE)
http://www.cisco.com/en/US/products/hw/phones/ps379/products_administration_guide_chapter09186a008066b89a.html#wp1088046
Hope this helps!
Rob
