Ross Hall Studio (ca. 1921). Horticulture students studying tree roots [Online image]. Washington State University Libraries Manuscripts, Archives, and Special Collections.
https://content.libraries.wsu.edu/digital/collection/p16866coll18/id/360

Below are links to some useful resources that tend to have open access/public domain images. Do remember, different images and websites may have different license and use policies. 

Agriculture, Horticulture, and Land-Use Photographs

National and International Collections 

• Library of Congress Image Catalog - Thousands of images from the U.S. Library of Congress.
  • See also: Library of Congress Online Photo Catalog - Presents over 1,300 items with a over 4,000 images and a date range of 1824-1931.
• USDA Agriculture Research Services Image Collection - Many fantastic, and current, agriculture and horticulture images.
• USDA National Agriculture Library (NAL) Image Collection - The NAL provides many images across numerous subjects. 
  • See also: NAL "Art & Images."
• University of Nebraska Institute of Agriculture and Natural Resources - Free Food-Related Photo Sources
• New York Public Library Image Collection - Thousands of images across many subjects.
• Bio Heritage Library Photo Collection - This link provides information regarding searching the extensive BHL historic image library.
• United States Geological Survey (USGS)  - A searchable archive of thousands of USGS historic photos.
• Natural Resources Conservation Service Photos & Multimedia - A large collection of images and animations from the NRCS. 
• USGS Multimedia Library - Explore our planet through photography and imagery, including climate change and water all the way back to the 1800s when the USGS was surveying the country by horse and buggy.

Regional Content

• Washington State University Libraries Digital Collections - WSU Libraries Digital Collections provide access to the academic and cultural heritage of Washington State and the Palouse Region.
• Washington State Library Photograph Collection, 1851-1990 - This collection consists of 5,274 images of various subjects related to Washington's history, people, geography, and economic development (1851-1990).
• University of Washington Online Special Collections - Digital collections covering a wide range of subjects pertaining to regional history.
• Whitman County (Washington) Rural Heritage Collection  - Many historical images of agriculture around the state and on the Palouse.
• Idaho State Archives / Historical Society - Collection of Idaho historic digital collections.
• Idaho Transportation Photo Collection - 30,000+ historic pictures.
• Idaho State Archives Photographs - The photograph collection includes over 500,000 images, dating from the territorial period to present day.
• The Plateau Peoples' Web Portal (Collections) -  The Plateau is a  Peoples' Web Portal collaboration between many northwestern and plains tribes and the WSU Center for Digital Scholarship and Curation. This portal serves as an extensive repository of cultural materials including many photographs. 

See also:

• The Internet Archive (archive.org) - An extensive multidisciplinary resource that includes books, photographs, movies, ephemera, and much more. For example, here's a publication from 1901 about the flora of the Palouse.
• Unsplash - Dedicated to sharing open photography. The website hosts over 207,000 contributing photographers and over 2 million photos.
• You can also locate images using Google Images - Creative Commons Search. This searches for images utilizing a Creative Commons license. For example, here's a search for images of "Palouse" which have a Creative Commons License. 
• The Creative Commons has a multimedia search option via this link.

Maps

• Washington State University Manuscripts and Special Collections Early Washington Maps Collections - Over 1200 historic maps of Washington in high resolution.
• Washington State Library Historical Maps - A wide range of different types of historic maps regarding Washington State. 
• USGS Map Collection - A wide range of maps ranging from topographic to land use. Maps are searchable but the website can be cumbersome.
• USGS Mineral Resources Online Spatial Data - Interactive and searchable maps regarding natural resources
• USGS National Geospatial Program - Representing searchable historical geospatial data regarding topography, natural landscape, and manmade environments of the United States.
• Library of Congress Map Collection - Large searchable collection of numerous types of maps.

Bauer, S. (2000). In Salinas, California, geneticist Robert Lewellen and technician Jose Orozco evaluate sugar beet breeding lines for disease resistance [Online image]. USDA Agricultural Research Service. https://www.ars.usda.gov/oc/images/photos/aug00/k8991-1

Free Image Editing Software

GNU Image Manipulation Program (GIMP) - GIMP is a free and open-source graphics editor. It can be used for image creation and editing, changing and saving different image formats, free-form drawing, and much more.

PIXLR - A free (basic package) web-based image editing application.

*There's lots more! These are just two applications that I use.