How many you will need to show a difference depends on how big the difference that you are trying to find is (if it exists).
I would think 30 of each type (90-100 total) would be a good start.
About a few of the other points : maybe you can make the degrees from some hard to check place while still being believable...
You are planning on sending 3 cv per person?
If not, then you should juste use one cv that would make things easier.
In both cases you should just study one variable, the photo.
tell us how it goes...