Flashing official non-AT&T firmware for the Lumia 925 should be done using Nokia Care Suite. There's a (HUGE!) thread on un-branding WP8 devices by flashing a different official ROM to them.
Technically you *could* un-brand an AT&T ROM via JTAG, but it wouldn't be worth the time.
Technically you *could* un-brand an AT&T ROM via JTAG, but it wouldn't be worth the time.