I have seen this come up a few times and I have seen folks recommend Cisco Phone Designer. This is an application Cisco offers for free download. It allows you to customize ring tones and screen background from an easy to use interface. Unfortunately, it is a user tool not an admin tool. Meaning, it only allows you to push to one phone and not to all phones, which I think is what you are looking for.
That being said, the Cisco Phone Designer tool uses an API method on the Cisco IP phones that you could try to tap into. The method is called "setBackground" and I believe it is an undocumented method. I stumbled across it by perusing the Cisco developer forums:
I have tried this out and it works as advertised. You have to make sure that "Phone Personalization" is enabled for the device(s) you wish to modify. Of course, you will also need to develop a script/tool to push the changes to the phones. If you are comfortable with scripting then this isn't too difficult. If you aren't comfortable with scripting then you may need to pull in a resource who is.
Right now, I am not sure of an alternate method to force a phone to pull a specific background. I have looked and I had to resort to writing my own method using info from the link I provided above (and some trial and error).
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