My first thought would have been the network. You say the two Desktops are not networked, but does the PC downstairs wirelessly connect to the FIOS router? If so, they are networked, even if you cannot share files between them.
However, on 2nd thoughts I believe this is to do with Google itself. When that option to have a background image was introduced I was told I had to sign up to Google to enable it. I declined, but if you have signed up then that may be the reason. You will be logged in using the same Google account on both machines, so as soon as they load up your home page, they apply the background according to your log in details.
I have a curious situation. My desktop pc is hardwired to my FIOS router. I use Google as my home page. I like to change the background photos they offer every few days. I have another desktop computer one floor below that gets the wireless signal. I use Google as my home page on that pc as well. I just noticed that when I change the background photo on the upstairs pc the downstairs pc has the new photo as the Google background also. I don't have a network set up. Anybody know why and how the photo could change on the pc downstairs? Thanks