From the ARTstor Welcome Page you can Browse or Search collections.
Browse by Collection:
To browse a collection, simply click on the collection name (third on the bulleted list under the Browse heading on the ARTstor home page). You can also do this by choosing "Collection" from the "Browse ARTstor by" choices under the "Find" tab on the ARTstor home page. You will be taken to a collection page with a list of expandable categories. Click on a category or subcategory title to browse images.
About the Collection:
The ARTstor Digital Library is both a broad and deep resource for teaching in the arts. The collection currently contains tens of thousands of high-quality images with authoritative cataloging from source collections. You can now explore its depths by browsing via source collections, i.e., the archives, libraries, museums, individuals, and institutions that contribute the various collections which comprise the Library. For example, you can browse by Metropolitan Museum of Art: subdivision Prints: subdivision Italy and get images of objects which match that grouping.
Browse across Multiple Collections:
You browse across collections by Geography or by Classification. For example, you could browse by Photographs: subdivision Indonesia, or conversely, browse by Indonesia: subdivision Photographs. (The Humanities and Social Sciences classification relates to material culture objects and the like. The Maps, Chart and Graphs classification is a good one to remember for historical geography.)
More on browsing ARTstor (from the ARTstor Help wiki)