I have the Bushnell Trophycam. It takes <2ma of power so the AA batteries could last for many months. The Passive IR is very sensitive to even a large bug or a mouse 30m away. Too small to see in a picture. It depends on the outdoor temperature.
The PTP library will work with most Canon's even the cheap consumer models. I have the 500D T1i DSLR which was $200. The quality of your WGI image and my Bushnell are OK, but they do not compare to a DSLR which costs about the same without any additional hardware. With a USB extender I can view what the camera is seeing from 300m away. Would you be interested in those details?
Otherwise we can use XBee to view the thumbnails while the camera takes the pictures under control of the Arduino with PIR sensor.
Certainly a Uno can press a button, but I don't understand how it will trigger and why in your case.