Cisco has a great guide on how to change the bacground image on a 7970 or 7971 phone. One thing the tech note does not mention is 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 VERY 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_note...