Before deciding to use a photo obtained from an internet source you should consider the rights of fellow artists and implemented copyright laws to protect the artist. As a painter or photographer it is generally considered poor etiquette in the art world to capitalize on another work without express permission. It is also against the law.
Whether the original artist has placed a © on his/her work or not it is not required by law and the time in which these rights apply may be longer than you might think. The latest copyright news and legislation can be found at the Sonny Bono Copyright Term Extension Act where it had been passed by the US House and the Senate and signed by President Clinton October 27, 1998 that “Any copyright still in its renewal term at the time that the Sonny Bono Copyright Term Extension Act becomes effective shall have a copyright term of 95 years from the date copyright was originally secured”
Why bother using images and which ones can I use from the internet? For artistic reference of course, for copying no. As an example you could probably make use of a graphic description of an animal or bird to place in a landscape but, most likely you will not get away with painting the exact image that was photographed unless the photographer permits you to do so. And don’t even think about prints.
Of literally millions of images available, it all depends on your intent. The general rule is, to use anyone’s image with a registered copyright or not you will need permission to use it if you yourself intend publish or take credit for it in commerce. The copyright laws give the original creator the final say. Also keep in mind that money has nothing to do with credits when comes to an expressed concept. So you should only contemplate taking reference from images that you find available online as “Public Domain” and always be sure about the status of a copyright before using it as you will find in some of the links provided below.
For more information about Copyright Laws go to the source and visit:
Other informative links about copyrighting:
List Where Reference Photos Are Available Online
Library of Congress – Prints & Photographs Online Catalog
Pen Ink Watercolor Sketching – Free Photo Collections for Sketching, Download Pen, Ink and Watercolor Wash Drawing & Sketching Tip Sheets, Tutorials, and Practice Sheets in PDF format.
Liam’s Pictures from Old Books – free images scanned from more than 160 different old or rare books
DOMAI – Tasteful Nude Photos (available to use with consent and terms of photographer)
US Fish and Wildlife Service’s – online digital library
National Oceanic and Atmospheric Administration -Photo Library Collection
WWII in Color – A great collection of World War II photographs
GRIN Images – NASA collection of over 1000 images
National Park Service – public domain digital images of national parks, monuments, battlefields, historic sites and related areas
Best Photos US – Millions of Free Photos