In a centralized system, all the images are held in one spot. If I were to walk up to a camera, my picture would be taken and it would be compared to the centralized database of millions and millions of images.
A decentralized system would be something like what we have on our passports, where there's a digital passport image encoded on the passport. When I use that, a picture is taken. The digital image on my passport gets compared with the one that was just taken. The comparison happens in that way.
There are still security risks, because the security on the digital radio device encoded on the passport can have breaches, but then you breach one passport. Not all of the biometric templates are in one place, where it's a lot easier to just grab the entire database.