Playing audio over bluetooth also wakes things periodically, but it uses almost no power. With a similarly competent design photo transfer should be very battery efficient.
No, most(if not all) bluetooth controller has audio offload, basically the cpu does not involve(or very little) when transfering data from storage to radio.
Don't you mean yes bluetooth controllers have audio offload? "similarly competent design" means you do the exact same thing in the other direction. Instead of rarely waking up the CPU to coordinate sending, you rarely wake it up to coordinate receiving.
