Web User Interface
Preferences - General Information (Section)
Phone User Interface
Initial Firmware Version
If you want to use your own background image on the phone, then a reachable valid URL to an image has to be set in this setting. As soon as the setting is changed, the background image will be replaced. You can change this setting via setting URL or over WUI->Preferences. If this setting is empty or the URL or the image is wrong, then the default phone background image will be used.
Important note for firmware V10: In case you configured the URL and it looks like all should work, but the phone user interface just doesn't show your desired image, please check the setting background color. Change the current / default alpha value to 0, so the desired background image does not get overlaid anymore.