Hi Achim,

The default background image is part of the phone firmware and thus can't be changed.

The Cisco IP Phone firmware includes a default background image. This image is not defined in the List.xml file. The default image is always the first image that appears in the Background Images menu on the phone.

From this doc;

http://www.cisco.com/en/US/products/hw/phones/ps379/products_administration_guide_chapter09186a00804cefff.html#wp1030672

**You will probably want to use the Idle URL method listed here;

Idle

Enter the URL that displays on the Cisco IP Phone display when the phone has not been used for the time that is specified in Idle Timer field. For example, you can display a logo on the LCD when the phone has not been used for 5 minutes.

Leave this field blank to accept the default setting.

Idle Timer (seconds)

Enter the time (in seconds) that you want to elapse before the URL that is specified in the Idle field displays.

Leave this field blank to accept the value of the Idle URL Timer enterprise parameter.

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00801ecfc7.html#1121361

Heres how on CCM;

Method 1:

Choose System > Enterprise Parameters so that the change is propagated to all of the phones from the Cisco CallManager Administration page.

In the URL Idle field, enter http://IP_address_of_CallManager/CCMCIP/filename.xml.

In the URL Idle Time field, enter any positive value in seconds.

Method 2:

Choose Device > Phone under the Cisco IP Phone configuration so that only this phone displays the graphics from the Cisco CallManager Administrator page.

Choose the phone/phones on which you want to put the display.

In the Idle field, enter the path http://IP_address_of_CallManager/CCMCIP/filename.xml.

In the Idle Timer field, enter any positive value in seconds.

From this good doc;

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_tech_note09186a00801c0764.shtml#topic4

Hope this helps!

Rob