Q:

Like masthead, motto, logo

What is the word for an image and/or phrase that might be painted or hung on the main wall of an organization's lobby? Not their "logo" but a deeper, more complex, more internal symbol. That's representative of the organization's identity to its members.
I'm thinking of the words "masthead" and "motto" as possible choices.

A:

In book publishing, many companies have an image associated with their books and their publishing house—for example, the Penguin Books penguin, the Pocket Books kangaroo, the Modern Library torchbearer, the Knopf borzoi, the Simon & Schuster seed sower, the Viking longboat, the Pantheon temple, the Random House mansion, the Oxford university seal, the Merriam-Webster wreathed MW, the Doubleday anchor, the Houghton Mifflin dolphin, and the Dent dolphin-and-anchor. These images/logos/devices are called colophons. 
Merriam-Webster's Eleventh Collegiate Dictionary offers the following relevant definition of colophon:

colophon n 2 : an identifying mark, emblem, or device used by a printer or a publisher

But as this definition suggests, the term colophon is primarily used within the publishing industry. 
Another possible term is insignia. Again from the Eleventh Collegiate:

insignia n 1 : a badge of authority or honor 2 : a distinguishing mark or sign

In U.S. periodical publishing, masthead refers to the list of employees who contributed to the writing, editing, design, and production of the publication in question, so that wouldn't be a good choice for what you're talking about.

