Two I can think of; speed and error resilience.
Accessing through a virtual drive mounted ISO is inevitably going to be slower than accessing files directly. For opening single photos in a viewer then it's probably going to be unnoticeable though.
As far as errors go any corruption to the ISO file may well leave the entire lot inaccessible. Of course most hard drive errors will be much more serious than a little, non-recoverable corrupted data, but let's say you do get a small amount of corruption on your disk.
With folders containing individual files this would only leave any files within which the data was corrupted as unreadable (unopenable as images).
However the same small data corruption anywhere in the ISO file will most likely leave the entire ISO image un-mountable and hence un-readable.
However if you have also used the ISO to burn a backup disk kept safely in a drawer somewhere then it doesn't matter (and is unlikely with modern drives anyway).