FYI that is how Microsoft Outlook shows Digital Signatures.

PKI is not a single entity, and it doesn't need a single icon associated with it. PKI is used for Digital Signatures, SSL, and encryption among many other things- and hence a generic term Public Key Infrastructure to indicate any implementation of Public Key Cryptography.

My all-time favourite illustration from that site has always been the energy levels of the band structure, for varying wavevector...

No kidding. Thought some guy in my research group was nuts when he told me Britney knew a lot about band structure of semiconductors. But turns out, it's true!