Regardless of where the screensaver is stored, each device has have it assigned in its phone applications.
In v8, with the 6900 phones, the phones have a screensaver configured by default, so if you want to change it for all phones, you just upload your new screensaver with the correct filename, all phones will pretty quickly start displaying the new one.
With the 5300 phones, no such luck. With 1000 users, you need a patient monkey....