Believe it or not, books are one of the best sources of images you will be able to find.
Not only will images be high-quality and culled from archives and collections that might otherwise be difficult to access, they will also be hand-selected to reflect the subject matter that interests you.
Books of images will often include essays, memoirs, and other descriptive text to help contextualize what you see.
You can tell whether or not a book contains images by looking for the abbreviation "ill." in the catalog record. Often, the record will even tell you how many images or "plates" the book contains.
To find books that are primarily composed of images, you will want to search using LC subject headings ending with the phrase "Pictorial works." (See below...)
Library of Congress subjects to explore in Colby's catalog, OneSearch, CBBcat, MaineCat, NExpress, and WorldCat...
Primarily pictorial books within these subject designations include...