I'm currently reading The Audiovisual Cataloging Current edited by Sandra K. Roe. It includes a paper giving an overview of the TGM.
"The Thesaurus for Graphic Materials I: Subject Terms (TGM I) provides a substantial body of terms for subject indexing of pictorial materials, particularly the large general collections of historical images"--I.A.
The other section of TGM is The Thesaurus for Graphic Materials II: Genre and Physical Characteristic Terms (TGM II). This section contains over 600 terms describing distinctive categories of material: an established class of pictorial types, a vantage point or method of projection, or intended purpose. Some indicate characteristics of an image's creator or a publication status or occasion. Others imply a subject but also designate a method of representation. Physical characteristic headings designate graphic materials distinguished by production processes or techniques, production stages or versions, instrument employed, markings, shape and size, and other physical aspects of graphic materials. (Adapted from the Scope and Purpose note)