If you're interested in becoming one, they used to say in the forum that the number one rule was "don't ask"

I've met some of them in person (at the iStockalypse HQ) - many are fellow contributors (all exclusives). You can see the silver badge with the I in their list of icons to know if someone is an inspector.
No idea about pay or training, but they span the globe (don't think there are any in Antarctica but pretty much everywhere else). At one point the video inspectors were at HQ only, but that may have changed (I don't keep up with video much).
There had been an article "Meet the inspectors" ages ago, but I think they're now up to 100+?