There are many image sources available on the internet, at some universities and other institutions. Some provide limited access to copyright free use of images. Most require some form of permission and a fee for image use.

American Landscape and Architectural Design, 1850-1920 Harvard Univesity's collection of more than 2,000 slides, plans, maps, & models provides a visual history of American buildings and landscapes built betwen 1850-1920.  Images of cities, specific buildings, parks, estates and gardens are included.  

American Memory The Library of Congress'collections of digitized documents, photographs, audio, moving pictures, and text from the Library of Congress' Americana collections.

Botanical Image Database--University of Basel

Cactus and Succulent Plant Mall Picture Gallery

Cities/buildings Archive "Begun in 1995, the Database was conceived as a multi-disciplinary resource for students, faculty, and others in the academic community. It has grown steadily since then, with contributions from a wide range of scholars, and now contains over 5000 images ranging from New York to Central Asia, from African villages, to the Parc de la Villette, and conceptual sketches and models of Frank Gehry's Experience Music Project. These have all been scanned from original slides or drawn from documents in the public domain. They are freely available to anyone with access to the Web for use in the classroom, student study, or for individual research purposes."

History of Landscape Architecture A selection of digitized colour photographs of gardens and 18th and 19th century Western landscapes, paradise gardens, Islamic, Spanish, Moghul, medieval gardens.

History of landscape horticulture A powerpoint collection of slides of gardens from earliest times

Images of Medieval and Renaissance Gardens
Medieval Gardens. Monastery Influences. Royal Influences. Renaissance Gardens. Fences and Enclosures. Arbors. Raised Beds. Grafting. Training. Gardening ...

International Bulb Society : Gallery of the world's bulbs "A pictorial and cultural guide to the world's geophytic plants. All images and text are copyrighted © by the contributors"

Monumental Land Art of the United States Land Art, "also called Earth Art , Environmental Art or Earthworks, is primarily a sculptural movement encompassing creative work that integrates physical or conceptual elements of the landscape into the finished piece." Includes satellite images, topographic maps and links to sites for actual works

S.P.I.R.O. "Online public access to about one sixth of the U Cal Berkeley’s Architecture Slide Library collection of over 200,000 35mm slides.. Subjects include: architecture, landscape architecture and urban studies".

Vilar: Virtual Landscape Gallery A collection of over 1700 photographs of European landscapes and gardens. Searchaablein French or English by century (15th-20th), site name , garden feature etc.

Hort.net 3162 images, 1592 plants (as of December 31, 2002)

RIBApix growing database dedicated to providing you with exceptional and unique images from the collections of the British Architectural Library at the Royal Institute of British Architects, the world’s most extensive visual archive devoted to architecture"

History of Landscape Architecture is a comprehensive collection of photographs and other images created to to support lectures on the history of landscape architecture at the University of Oregon. The images are organized into several categories, among them: Gardens Agriculture, Egypt, Greece, Roman, Primitive, Paradise Gardens, Islamic, Spanish, Moghul, Medieval Gardens, Forest, French Gardens, Pastoral, English Gardens, Parks Movement, America 18th-19th Century. Not an easy site to use as there is no text and the site is not searchable, although each image is captioned.

Univ. of Manitoba - Direct Link to Architecture Library - slide collection but no subject
guides  http://umanitoba.ca/libraries/units/archfa/index.html