What is Open Access?
"Open Access stands for unrestricted access and unrestricted reuse." (this definition is from PLoS (Public Library of Science), an open-access peer-reviewed journal). Open access doesn't mean that the copyright holder has given up their copyright -- items with no copyright holder or whose copyright has expired are in the "public domain."
Creative Commons is on a mission to make open access content more accessible and understandable by providing a set of licenses that can be used for open access content. The copyright holder gets to choose the license they want to apply to their own work.
Images from the sources listed below are generally available for use on the open web. Remember:
Image sources that use Creative Commons licenses:
Image sources that don't use Creative Commons licenses
These are fantastic image sources, but it can be harder to determine what rights you have for each individual image. Be sure to read the permissions carefully to understand if you're allowed to use and modify the images.
Images and figures are the most frequent victims of accidental plagiarism -- remember that if you use an image you didn't create, you have to provide a citation (even for very small images!). The citation should be easily accessible in the context of the image's use (within a PowerPoint presentation, on a web page, in a paper, etc.).
Image citations should include the following information at a minimum:
It is also useful to include date, culture, and rights information, if known.
Check out the Creative Commons page on How to give attribution.
To cite figures in ACS style, see Chapter 14, pages 366 & 367, of Coghill and Garson; The ACS Style Guide; American Chemical Society: Washington, DC, 2006.
Templates for citing a figure in ACS style -- these are designed for citing figures from peer-reviewed articles, so the information you include is different than what you'll include for images you find online.