Min power draw - 3.4 Watts
Max power draw - 12.5 Watts
Average power draw 8 Watts
firstly, did you just average 3.4 and 12.5 to get the number 8? - does it really spend half the time at min power and half at full, or is it more like 95% of the time idle and 5% at max power?
Furthermore, your above setup is with a hard drive - would your battery powered setup also have a hard drive? If not, that could be a 3 watt saving.
(Plus perhaps you only take photos max 16 hours a day, not 24.)
Also, I would turn the Pi and the camera off when they're not active. Have a microcontroller - which would have a similar power draw to a wrist watch - and every 10 minutes it turns on your camera, and every 2 hours it turns on the Pi, it does nothing else.
The camera could have an autorun script to say - 'if no usb power, take a photo and shut down, else if there is usb power, sit and wait for the pi to talk to me' - and the Pi could have a startup script to say 'download photos from camera, turn camera off, upload photos to web, then shutdown.'
(You would need to solder wires to the camera's power button to do this, so that an electrical signal from the microcontroller would be able to simulate a physical press of the button.)
So all these possibly mean the power consumption is drastically reduced - if the Pi is now on for 2 minutes every 2 hours instead of 120 minutes, that's 95%+ power saved right there. The camera could also be mostly switched off, depending on the time interval. (If the time interval is very short, then any increased power consumption for startup and shut down could outweigh the saving from completely turning off.)
Then even a modest usb battery pack should suffice for a few days - one with 10000mAH capacity has an energy capacity of 50WattHours (5 volts x 10 000 milliamp hours), so if your *average* power consumption is now 0.5 watts, it would last for 100 hours.