Images on the open web are subject to copyright law in the same manner as any other creative work; there is no guarantee that an image is legally available for re-use just because it is freely accessible on the web.
That said, there are many cases in which copyright law permits re-use:
1. The image is a public domain work. Generally, anything published in the U.S. before 1923 is in the public domain. For more info, check out the Peter Hirtle's Copyright Term and the Public Domain.
2. The image is available under a Creative Commons license. CC images are labeled as such. When using a CC image, be sure to provide proper attribution to the source.
3. The image is otherwise made available for re-use by the content provider. Some websites permit you to re-use their images on your own website, as long as certain conditions are met (e.g. noncommercial use only). In these cases, you can find out whether re-use is permitted by looking at the website’s Terms & Conditions.
4. The image is copyrighted, but re-use qualifies as Fair Use. In the context of using images on a website, you have a stronger Fair Use argument if you are directly commenting on or critiquing the image, or if you are using the image in a way that is transformative.
5. You have permission from the copyright owner. For more information, see Getting Permission.
Image credit: The Internet by Mikael Altemark via Flickr.com 
It can be tricky to determine the creator of a web image. To make giving credit easy, look for images that give you enough information to attribute them.
At minimum, do your best to:
Generic Image Credit Format:
Title by Creator Name, via Source (License Type).
Examples:
Image credit: Jack in Sink (again) by J. Smith, via Flickr.com (CC NC-BY 2.0).
Image credit: 13694 by A. Mills, via CDC.
Image credit: Mrs. Claire... by D. DeMarisco, via Library of Congress, Prints & Photographs Division, NYWT&S Collection.
Here are several excellent sources for images in the public domain and creative commons-licensed content:
Creative Commons Search - Allows you to search for CC-licensed material by media type
College Art Association Image Sources - Links to free, restricted and unrestricted, image banks.
Flickr Advanced Search - Choose “Only search within Creative Commons-Licensed Content.”
Google Advanced Image Search - Use the “Usage Rights” field to limit by license type.
Library of Congress: American Memory - A free “digital record of American history and creativity.” Library of Congress: Prints & Photographs Online Catalog - Photographs, prints, drawings, posters, and architectural drawings, and more.
NASA Image Library - A collection of over a thousand images of significant historical interest scanned at high-resolution
NGA Images - Public domain artworks from the collections of the National Gallery of Art.
Noun Project - Free clip art images requiring creator credit.
NYPL Digital Gallery - Illuminated manuscripts, historical maps, vintage posters, rare prints, photographs, and more, from the New York Public Library.
US Government Photos & Images - Public domain images by topic.
Wellcome Images - All images are made available under Creative Commons licenses.
Wikimedia Commons - Browse or search for freely reusable images.
Wikipedia Public Domain Images - List of public domain image sources on the web.
The Library provides MUW faculty and students with access to a number of great images, including the images available in EBSCO and Gale databases. These images are high resolution and free for use by anyone with a 950- number and access to Library electronic materials.
However, use of these database images are subject to license terms and conditions that limit how you may reuse the material. While you are free to view images on the database and even download them for your own study in many cases, you are not permitted to post database images to public websites, including your own personal websites or study sites, such as Study Blue. Doing so is a violation of the user terms of the database.
If you wish to use any images you find in your own scholarship, such as a thesis, dissertation, or article, you will need to contact the database owner for permission to republish the images. You can find out more in the section on this resource page from NYU Libraries.
