How does one actually transmit the images from that high up? I can't imagine you get cell signal up there.
I don't know what they used, it probably was on the Hambands. On our balloon, we transmitted APRS mostly for tracking and retrieval. They could have set up ATV or some other custom protocol.
How is this any different than any other embedded Linux device? Legitimately curious. The thing that sticks out about the RPi to me is the GPU. I suppose it has GPIO but doesn't every other hobby device like this sport them?