Exhibitions & Online Albums
Anna Atkins Cyanotypes, on NYPL's Flickr Commons site
Dawn's Early Light: The First 50 Years of American Photography (Cornell University Library)
Collection catalogs document materials in a museum, gallery or private collection. Usually, the catalog is in print form, but increasingly, this genre of publication is transforming into online databases of digital images and associated metadata about each piece.
To search for online collection databases, visit museum websites and look for links (often menu items) to Collections. Usually, you'll find highlights or an image database. Here is the Metropolitan Museum of Art's Collection link:
You can also try Google's advanced image search (it's under "settings") and try searching for any of these words: collection gallery museum database.