Cisco has a great guide on how to change the bacground image on a 7970 or 7971 phone. Note that you have to create the List.xml file. Copy the one in the technote into notepad for an example and name it List.xml - It is case sensitive. Then copy it to flash on your UC520.
The link to the technote is below.
http://www.cisco.com/en/US/products/sw/voicesw/ps4625/products_tech_note09186a008062495a.shtml
