I know that they are meant to represent repo type, but there is no key or legend anywhere, not even in the documentation, and Googling comes up empty. I can't even cross-reference with the repo info page, because that page only displays the package type, not the repo type.
I only know of 3 types of Artifactory repos (local, remote, virtual), so I'm confused as to why there are so many icons.
You can find some sort of a legend when you press on "Repository Types" at the top line of the "Artifacts" view:
The missing icon belongs to Build-Info Repository.
Read here about Federated Repositories.