Hi, I found no docs on it, however a sniffing session unveiled the secret.
Just follow the instructions for customizing the 7975 background and change the size from 320x216 to 800x600 (everywhere, size of the image, name of the directory, etc).
I also created the thumbnail image but it seems not to be used by the 7985.
Here's the 7975 instructions for reference :
http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/7975g/6_0/english/administration/guide/7975cst.html#wp1088046
HTH
Alberto