Collections are arranged into groups for searching. Click on a group name to search the collections in that group. A collection may include some content outside of the group's definition, and a collection could appear in more than one group. Click the title of a specific collection to search it alone. Sometimes it is advantageous to search a single collection. For example, search field and sorting options are often specific to the collection, allowing for a more refined search.

Some collections have a very narrow focus. Other collections are broader, and may encompass many sub-collections. Occassionally an image appears in multiple collections, though generally there is a single authoritative source for each image.