This issue has been discussed both within the TV series and the prose novels that lead on;
Simply put, a weeping angel can exist as an independent entity within a camera image or video footage. If you view that footage, the angel becomes frozen but as soon as it's not being observed, the Angel can move freely within the frame as well as exit the monitor you're using to observe it on.
We see this most clearly in the TV episode "Time of Angels"
AMY: Doctor Song? Did you have more than one clip of the Angel?
RIVER: No, just the four seconds.
(The Angel is now looking straight at the viewer, hands completely
lowered. The time stamp has moved as far as 00:11:28:04 before jumping
back to 00:11:24:23. When Amy looks at the monitor again, it is closer
to the camera. The module door closes and locks behind her.)
(Amy tries turning off the monitor, but it comes back on again, and
again, and again. She moves close to the monitor.)
AMY: But you're just a recording. You can't move. (She tries to pull
out the lead to the monitor. When she looks back up, the Angel is up
close to the camera, but the time stamp is still running through the
same four seconds.)
(Amy cannot open the door. The Angel's mouth is open.)