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.