Is there, in fact, any close-up photography of Saturn's rings, showing individual pebbles/rocks?
The rings are about 3% solid in the densest parts, but this translates to a separation between 30cm particles of about 1metre.

There are no images Because approaching the dense part of the rings would be extremely difficult. A probe would be travelling at orbital speeds, and even a small percentage difference in speed would translate to a large absolute difference. As the probe would also be in orbit, it would be extremely difficult to find an orbit that doesn't collide catastrophically with ring particles.

Cassini did travel through one of the gaps in the ring system during Saturn orbital insertion. It survived 100000 dust impacts in 5 minutes.

Nature has some artist's impressions of the ring particles.