If you want to save battery power, or trigger an alarm on Uno, buy a Grove serial JPG camera.
If you want to save the images to SD, without an RF transmitter, serial JPG is the answer.
It can sleep and only transmit when there's movement.
You can measure this with PIR or any other sensor, in addition to the camera itself.
Put all 3 of them to sleep for an interval:
Uno
XBee
Grove camera
This would use much less power than an always on 2.4Ghz sender.
Mine uses 200ma compared to an average of 2ma for Grove.
I suppose you could put any transmitter to sleep to save power.